Iron(II) catalysts containing tridentate PNP ligands, their synthesis, and use thereof

ABSTRACT

The application describes catalytic materials for hydrogenation or asymmetric hydrogenation. In particular, the application describes iron(II) complexes containing tridentate diphosphine PNP ligands useful for catalytic hydrogenation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a national stage application of PCT/CA2015/050008, filed on Jan.8, 2015, which claims priority to U.S. Patent Application Ser. No.61/925,021, filed on Jan. 8, 2014. This application claims the benefitsand priority of these prior applications and incorporates theirdisclosures by reference in their entireties.

FIELD OF THE INVENTION

The present application pertains to catalytic materials forhydrogenation or asymmetric hydrogenation. More particularly, thepresent application relates to iron(II) complexes containing tridentatediphosphine PNP ligands useful for catalytic hydrogenation.

INTRODUCTION

Current research in the field of catalytic hydrogenation has movedtowards developing complexes that do not employ platinum group metalsgiven their low abundance and toxicity, which makes them undesirable forsome applications [Catalysis without Precious Metals; Bullock, R. M.,Ed.; Wiley-VCH. Hoboken, N.J., 2010]. Several catalysts based on 3dmetals have been shown to be competitive with precious metal basedcatalysts for asymmetric reduction of unsaturated bonds [Tondreau, A.M.; Darmon, J. M.; Wile, B. M.; Floyd, S. K.; Lobkovsky, E.; Chink, P.J. Organometallics 2009, 28, 3928-3940; Monfette, S.; Turner, Z. R.;Semproni, S. P.; Chirik, P. J. J. Am. Chem. Soc. 2012, 134, 4561-4564;Sylvester, K. T.; Chirik, P. J. J. Am. Chem. Soc. 2009, 131, 8772-8774;Wu, J. Y.; Stanzl, B. N.; Ritter, T. J. Am. Chem. Soc. 2010, 132,13214-13216; Dong, Z. R.; Li, Y. Y.; Yu, S. L.; Sun, G. S.; Gao, J. X.Chin. Chem. Lett. 2012, 23, 533-536; Vasudevan, K. V.; Scott, B. L.;Hanson, S. K. Eur. J. Inorg. Chem. 2012, 4898-4906; Zhang, G.; Scott, B.L.; Hanson, S. K. Angew. Chem. Int. Ed. 2012, 51, 12102-12106; Harman,W. H.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 5080-5082; Federsel,C.; Ziebart, C.; Jackstell, R.; Baumann, W.; Beller, M. Chem. Eur. J.2012, 18, 72-75; Werkmeister, S.; Fleischer, S.; Junge, K.; Beller, M.Chem.—Asian J. 2012, 7, 2562-2568]. Other systems based on enzymes[Huisman, G. W.; Liang, J.; Krebber, A. Curr. Opin. Chem. Biol. 2010,14, 122-129; Savile, C. K.; Janey, J. M.; Mundorff, E. C.; Moore, J. C.;Tam, S.; Jarvis, W. R.; Colbeck, J. C.; Krebber, A.; Fleitz, F. J.;Brands, J.; Devine, P. N.; Huisman, G. W.; Hughes, G. J. Science 2010,329, 305-309; Ringenberg, M. R.; Ward, T. R. Chem. Commun. 2011, 47,8470-8476; Matsuda, T.; Yamanaka, R.; Nakamura, K. Tetrahedron:Asymmetry 2009, 20, 513-557], or metal-free compounds, have also shownpromise [Stephan, D. W.; Greenberg, S.; Graham, T. W.; Chase, P.;Hastie, J. J.; Geier, S. J.; Farrell, J. M.; Brown, C. C.; Heiden, Z.M.; Welch, G. C.; Ullrich, M. Inorg. Chem. 2012, 50, 12338-12348;Rueping, M.; Dufour, J.; Schoepke, F. R. Green Chem. 2011, 13,1084-1105; Sumerin, V.; Chernichenko, K.; Nieger, M.; Leskela, M.;Rieger, B.; Repo, T. Adv. Synth. Catal. 2011, 353, 2093-2110; Farrell,J. M.; Hatnean, J. A.; Stephan, D. W. J. Am. Chem. Soc. 2012, 134,15728-15731; Farrell, J. M.; Heiden, Z. M.; Stephan, D. W.Organometallics 2011, 30, 4497-4500; Mandi, T.; Heiden, Z. M.; Grimme,S.; Stephan, D. W. J. Am. Chem. Soc. 2012, 134, 4088-4091; Reddy, J. S.;Xu, B.-H.; Mandi, T.; Fröhlich, R.; Kehr, G.; Stephan, D. W.; Erker, G.Organometallics 2012, 31, 5638-5649].

In recent years, a variety of iron-based hydrogenation catalysts havebeen developed [Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282-2291;Junge, K.; Schroder, K.; Beller, M. Chem. Commun. 2011, 47, 4849-4859;Darwish, M.; Wills, M. Catal. Sci. Technol. 2012, 2, 243-255; Gopalaiah,K. Chem. Rev. 2013, 113, 3248-3296; Le Bailly, B. A. F.; Thomas, S. P.RSC Adv. 2012, 1, 1435-1445]. Morris et al. reported an iron complexthat was moderately active for asymmetric hydrogenation of acetophenoneunder basic conditions, with a turnover frequency (TOF) of 5 h⁻¹ and anenantiomeric excess (ee) of 27% (S) (M1 of FIG. 1). The authors foundthat a monocarbonyl derivative of M1 was a good asymmetric transferhydrogenation pre-catalyst for ketones [Sui-Seng, C.; Freutel, F.;Lough, A. J.; Morris, R. H. Angew. Chem. Int. Ed. 2008, 47, 940-943],and later, Beller et al. demonstrated that it also worked for activatedimines [Zhou, S.; Fleischer, S.; Junge, K.; Das, S.; Addis, D.; Beller,M. Angew. Chem. Int. Ed. 2010, 49, 8121-8125].

Morris et al. also developed highly active and enantioselective iron(II)catalysts, [Fe(P—N—N—P)(CO)(Br)][BPh₄], where P—N—N—P was tetradentateligand (S,S)-Ph₂PCH₂CHNC(H)PhC(H)PhNCHCH₂PPh₂, formed by condensation ofan (S,S)-diamine with phosphine aldehydes templated by iron(II)[Mikhailine, A.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2009,131, 1394-1395; Mikhailine, A. A.; Morris, R. H. Inorg. Chem. 2010, 49,11039-11044]. Morris et al. modified the ligand by substituting thephenyl substituents on phosphorus with alkyl or substituted phenylsubstituents to examine effects on catalytic behavior [Lagaditis, P. O.;Lough, A. J.; Morris, R. H. Inorg. Chem. 2010, 49, 10057-10066; Sues, P.E.; Lough, A. J.; Morris, R. H. Organometallics 2011, 30, 4418-4431];the resulting pre-catalysts were activated by base (KOtBu) to formbis-eneamido iron(II) complexes which were half reduced by isopropanol[Lagaditis, P. O.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2011,133, 9662-9665; Mikhailine, A. A.; Maishan, M. I.; Lough, A. J.; Morris,R. H. J. Am. Chem. Soc. 2012, 134, 12266-12280]. The pre-catalyst'sketone reduction was found to involved a bifunctional mechanism wherethe ligand was directly involved in catalysis [Mikhailine, A. A.;Maishan, M. I.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2012, 134,12266-12280; Prokopchuk, D. E.; Morris, R. H. Organometallics 2012, 31,7375-7385]. Said catalysts were efficient at reduction of prochiralketones by transfer hydrogenation with isopropanol, achieving TOFs of upto 55,000 h⁻¹, conversions of 98%, and enantioselectivity (ee) upwardsof 90% for one enantiomer of the alcohol, at room temperature[Mikhailine, A. A.; Maishan, M. I.; Lough, A. J.; Morris, R. H. J. Am.Chem. Soc. 2012, 134, 12266-12280]. The catalysts were also active andenantioselective towards transfer hydrogenation of certain activatedimines [Mikhailine, A. A.; Maishan, M. I.; Morris, R. H. Org. Lett.2012, 14, 4638-4641].

Morris et al. determined that said transfer hydrogenation of ketones wasoccurring as an equilibrium process, sometimes resulting in productracemization. Therefore, a catalytic system was sought that utilized H₂gas, thereby enabling irreversible hydrogenation of substrates withcomplete conversion to, and no racemization of, the product.

Kirchner et al. developed a synthesis for Fe(P—N—P)(CO)X₂ complexes,wherein P—N—P was a tridentate 2,6-(PiPr₂NH)₂C₅H₃N ligand, and X was CIor Br [Benito-Garagorri, D.; Alves, L. G.; Puchberger, M.; Mereiter, K.;Veiros, L. F.; Calhorda, M. J.; Carvalho, M. D.; Ferreira, L. P.;Godinho, M.; Kirchner, K. Organometallics 2009, 28, 6902-6914;Benito-Garagorri, D.; Puchberger, M.; Mereiter, K.; Kirchner, K. Angew.Chem. Int. Ed. 2008, 47, 9142-9145]. The authors found that when X wasCl, the cis isomer formed under solvent-free conditions, while the transisomer formed in solution; however, when X was Br, a mixture of cis andtrans isomers was always obtained. Kirchner et al. also reported aselective synthesis of trans-[Fe(P—N—P)(CO)₂(Br)][BF₄] complexes from amixture of cis- and trans-Fe(P—N—P)(CO)(Br)₂ isomers via use of a halideabstractor (such as AgBF₄) [Benito-Garagorri, D.; Alves, L. G.; Veiros,L. F.; Standfest-Hauser, C. M.; Tanaka, S.; Mereiter, K.; Kirchner, K.Organometallics 2010, 29, 4932-4942].

Work by Milstein et al. demonstrated that iron(II) complexes comprisingan achiral P—N—P ligand can be effective hydrogenation catalysts. Theauthors' iron(II) complexes, Fe{2,6-(PiPr₂CH₂)₂C₅H₃N}(H)(CO)(Br) (M2)[Langer, R.; Leitus, G.; Ben-David, Y.; Milstein, D. Angew. Chem. Int.Ed. 2011, 50, 2120-2124] and Fe{2,6-PiPr₂CH₂)₂C₅H₃N}(H)(CO)(HBH₃) (M3)[Langer, R.; Iron, M. A.; Konstantinovski, L.; Diskin-Posner, Y.;Leitus, G.; Ben-David, Y.; Milstein, D. Chem. Eur. J. 2012, 18,7196-7209], shown in FIG. 1, were found to be active for catalytichydrogenation of ketones under 4.1 atm H₂. Turnover numbers (TON) of upto 1720 were achieved for acetophenone hydrogenation, with TOF ofapproximately 430 h⁻¹ at 40° C. for M2; and, up to 1980 TON with a TOFof 300 h⁻¹ at 40° C. for M3. The latter complex did not requireactivation by base. A mechanistic investigation using experimental andDFT methods concluded that the catalysts operated by Milstein'sestablished aromatization-dearomatization of P—N—P ligands, which hadbeen previously demonstrated to occur with Ru analogues [Yang, X. Inorg.Chem. 2011, 50, 12836-12843; Gunanathan, C.; Ben-David, Y.; Milstein, D.Science 2007, 317, 790-792; Gunanathan, C.; Milstein, D. Acc. Chem. Res.2011, 44, 588-602].

Knölker's Fe(II) complex was found by Casey et al. to catalyze ketonehydrogenation via a bifunctional mechanism using H₂ gas under mildconditions [Knölker, H.-J.; Heber, J. Synlett 1993, 1993, 924-926;Knölker, H.-J.; Heber, J.; Mahler, C. H. Synlett 1992, 1992, 1002-1004;Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816-5817; Casey, C.P.; Guan, H. J. Am. Chem. Soc. 2009, 131, 2499-2507]. Berkessel et al.synthesized a chiral analogue of Knölker's complex by replacing a COligand with chiral phosphoramidite ligands (B1, FIG. 1) [Berkessel, A.;Reichau, S.; von der Höh, A.; Leconte, N.; Neudörfl, J.-M.Organometallics 2011, 30, 3880-3887], while Beller et al. added a chiralphosphoric acid as a co-catalyst (B2, FIG. 1) [Zhou, S.; Fleischer, S.;Junge, K.; Beller, M. Angew. Chem. Int. Ed. 2011, 50, 5120-5124;Fleischer, S.; Werkmeister, S.; Zhou, S.; Junge, K.; Beller, M. Chem.Eur. J. 2012, 18, 9005-9010]. The Berkessel system hydrogenatedacetophenone with UV irradiation under 10 atm H₂ at 25° C., with 9turnovers in 24 h to give 1-phenylethanol in 30% ee; the Beller systemproduced chiral amines in up to 96% ee under 50 atm H₂ at 65° C., with16 turnovers based on iron in 24 hours.

The above information is provided for the purpose of making knowninformation believed by the applicant to be of possible relevance to thepresent invention. No admission is necessarily intended, nor should beconstrued, that any of the preceding information constitutes prior artagainst the present invention.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide catalytic complexesthat are useful for hydrogenation or asymmetric hydrogenation. It isanother object of the present invention to provide iron (II) complexescontaining tridentate phosphorus-nitrogen-phosphorus (P—N—P) ligandsuseful for the catalytic hydrogenation or asymmetric hydrogenation ofketones, aldehydes and imines.

In accordance with one aspect of the application, there is provided acomplex of formula (I)

-   -   wherein:    -   a dashed line indicates that a bond may or may not be present;    -   each R¹ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₁-C₈        alkoxy, C₃-C₁₀ cycloalkyl, aryl, or heteroaryl, each of which        may be optionally substituted; or the two geminal R¹        substituents combine to form a C₂-C₄ linear alkyl diradical or        C₃-C₁₀ branched alkyl diradical, each of which may be optionally        substituted, such that the two R¹ substituents, together with        the phosphorus atom to which they are attached, form a ring;    -   each R² is independently aryl, heteroaryl, C₁-C₁₀ alkyl, C₂-C₁₀        alkenyl, C₁-C₁₀ alkoxy, aryloxy, or C₃-C₁₀ cycloalkyl, each of        which may be optionally substituted; or the two geminal R²        groups combine to form a C₂-C₄ linear alkyl diradical or C₃-C₁₀        branched alkyl diradical, each of which may be optionally        substituted, such that the two R² substituents, together with        the phosphorus atom to which they are attached, form a ring;    -   R³, R⁴, R⁵, R⁶, R⁸, R⁹, R¹⁰ and R¹¹ are each independently H, or        C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, or C₃-C₁₀        cycloalkyl, each of which may be optionally substituted; or, R³        and R⁴, R⁵ and R⁶, and/or R¹⁰ and R¹¹, together with the carbon        atoms to which they are attached, form a substituted C₅-C₁₀        cycloalkyl ring;    -   R⁷ is absent, H, AlH₃, or AlH₅;    -   each L¹ is independently H, BH₄, AlH₄, a halide, CO, an        N-heterocyclic carbene, OR¹², or NCR¹³, wherein R¹² and R¹³ are        independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, or        C₃-C₁₀ cycloalkyl, each of which may be substituted; or, one of        L¹ may be absent;    -   when R⁷ is AlH₃ or AlH₅, at least one of the H may bridge with        Fe to form a cycle together with the atoms to which they are        attached;    -   n is 0, +1, or −1, wherein, when n is +1, the complex further        comprises at least one non-coordinating anion, Y; and, when n is        −1, the complex further comprises at least one non-coordinating        cation, Z; such that the total charge of the complex is 0;    -   with the proviso that, when the nitrogen is singly bound to the        carbon attached to R⁹, each of R³ to R¹¹ are H, one L¹ is CO,        and the other two L¹ is are Br, or Br and H, or BH₄ and H, then        the R¹ and R² substituents cannot all be isopropyl; and    -   with the proviso that, when the nitrogen is singly bound to the        carbon attached to R⁹, each of R³ to R¹¹ are H, R⁷ is absent,        one L¹ is absent, one of L¹ is CO, and one of L¹ is H, then the        R¹ and R² substituents cannot all be isopropyl.

In one embodiment of the complex of formula (I), with the proviso that,when the nitrogen is singly bound to the carbon attached to R⁹, each ofR³ to R¹¹ are H, one L¹ is CO, and the other two L¹'s are Br, or Br andH, or BH₄ and H, the R¹ and R² substituents cannot all be cyclohexyl. Inanother embodiment of the complex of formula (I), with the proviso that,when the nitrogen is singly bound to the carbon attached to R⁹, each ofR³ to R¹¹ are H, R⁷ is absent, one L¹ is absent, one of L¹ is CO, andone of L¹ is H, the R¹ and R² substituents cannot all be cyclohexyl.

In accordance with one embodiment, there is provided a complex havingthe structure of formula (Ia):

In accordance with another embodiment, there is provided a complexhaving the structure of formula (Ib) or (Ic):

In accordance with another embodiment, there is provided a complexhaving the structure of formula (Id):

wherein X is a halide.

In accordance with another embodiment, there is provided a complexhaving the structure of formula (Ie) or (If):

In accordance with another embodiment, there is provided a complexhaving the structure of formula (Ig) or (Ih):

In accordance with another embodiment, there is provided a complexhaving the structure of formula (Ii):

In accordance with another embodiment, there is provided a complexwherein each R¹ is independently C₁-C₈ alkyl or C₃-C₈ cycloalkyl; or,alternatively C₁-C₄ alkyl or C₃-C₆ cycloalkyl; and, each R² isindependently aryl, or heteroaryl. In another embodiment, both of the R¹substituents are either isopropyl or cyclohexyl; and, both of the R²substituents are phenyl.

In accordance with another embodiment, there is provided a complexwherein R³ and R⁶ are each independently H, C₁-C₈ alkyl, aryl, orheteroaryl; or, alternatively H, C₁-C₄ alkyl, aryl, or heteroaryl; and,each of R⁴, R⁵, R⁸, R¹⁰ and R¹¹ is H. In one embodiment, R³ and R⁶ areeach independently H, methyl, or phenyl.

In accordance with another embodiment, there is provided a complexwherein R¹² is C₁-C₈ alkyl, C₂-C₈ alkenyl, C₃-C₁₀ cycloalkyl, aryl, orheteroaryl; or, alternatively, R¹² is C₁-C₄ alkyl, C₂-C₄ alkenyl, C₃-C₆cycloalkyl, aryl, or heteroaryl. In one embodiment, R¹² is methyl,ethyl, t-butyl, or t-amyl.

In accordance with another embodiment, there is provided a complexwherein the non-coordinating cation, Z, is an alkali metal cation, suchas K⁺, Na⁺ or Li⁺.

In accordance with another embodiment, there is provided a complexwherein the non-coordinating anion, Y, is a conjugate base of a strongacid, such as a halide, BF₄ ⁻, PF₆ ⁻, SbF₆ ⁻, NO₃ ⁻, ClO₄ ⁻, CF₃COO⁻,R¹⁴SO₃ ⁻, CF₃SO₃ ⁻, C₆H₅SO₃ ⁻, p-CH₃C₆H₄SO₃ ⁻, phosphates, TRISPHAT(Δ-or Λ-P(OC₆Cl₄O)₃ ⁻), carboranes, B(R¹⁴)₄ ⁻, or Al(R¹⁴)₄ ⁻, each of whichmay be substituted, wherein each R¹⁴ is independently an optionallysubstituted C₁-C₆ alkyl, aryl, heteroaryl, C₆H₃(CF₃)₂ and C₆F₅, halogen,pseudohalogen, C₁-C₈ alkoxide, or aryloxide. In one embodiment, Y is BF₄⁻.

In accordance with another embodiment, there is provided a complexhaving the formula of (I), (Ia), (Ib), (Ic), (Id), (Ie), or (If),wherein carbon C¹, C² or C³, or any combination thereof, is chiral, andthe complex is enantiomerically enriched, or a racemate. In anotherembodiment, there is provided chiral complexes:

In accordance with another aspect, there is provided a process forpreparing any one of the complexes (I), and (Ia)-(If), comprisingreacting a phosphine-aldehyde precursor of formula (II)

wherein R¹, R⁸, R¹⁰, and R¹¹ are as defined above,

-   with a phosphine-amine of formula (III)

wherein R², R³, R⁴, R⁵, and R⁶ are as defined above,

-   in the presence of-   an iron(II) compound,-   a CO atmosphere, and-   a strong base,-   to form a complex of formula (Ie), or to form a mixture of complexes    of formula (Ie) and formula (If),

wherein X is a halide, and

-   L¹ is CO;-   the complex of formula (Ie), or the mixture of complexes of formula    (Ie) and formula (If), is further reacted in the presence of-   a silver salt, and-   a CO atmosphere,-   to form a complex of formula (1d)

which is further reacted in the presence of a reducing agent to form thecomplex of formula (Ib) and/or (Ic)

wherein R⁹ is H,

-   which are further reacted with an excess of a primary, secondary, or    tertiary alcohol, to form the complex of formula (Ia)

wherein R⁷ and R¹² are as defined above.

In accordance with another embodiment, there is provided a processwherein carbon C¹ and/or C² of the phosphine-amine of formula (III) ischiral; and/or carbon C³ of the phosphine-aldehyde precursor of formula(II) is chiral; and, the compound of which they are a part isenantiomerically enriched, or a racemate.

In accordance with another embodiment, there is provided a processwherein the iron(II) compound is an iron(II) salt; or, an iron(II)complex. In another embodiment, the iron(II) salt is FeBr₂ or FeCl₂; or,the iron(II) complex is Fe(CO)₄Br₂, wherein Fe(CO)₄Br₂ is additionallyreacted in the presence of UV radiation to aid in formation of thecomplex of formula (Ie); or, to form a mixture of complexes of formula(Ie) and formula (If).

In accordance with another embodiment, there is provided a processwherein the strong base is KO^(t)Bu, the silver salt is AgBF₄, thereducing agent is LiAlH₄ or NaAlH₄, and the alcohol is MeOH, EtOH,^(t)BuOH, or ^(t)AmylOH.

In accordance with another aspect, there is provided a process forpreparing any one of the complexes (Ig)-(Ih), comprising reacting acomplex of formula (Ia)

-   -   wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R¹⁰, R¹¹, R¹² and L¹ are        as defined above,    -   in the presence of    -   a base, and

-   a H₂ atmosphere    -   to form a complex of formula (Ig) and/or a complex of formula        (Ih),

In accordance with another embodiment, there is provided a processwherein carbon C¹, C² or C³, or any combination thereof, of complexes offormula (Ia), (1g), or (1h) is chiral, and the complex isenantiomerically enriched, or a racemate.

In accordance with another embodiment, there is provided a processwherein the base is KOtBu, NaOtBu, Ph-CH(OK)CH₃, or NaOMe.

In accordance with another aspect, there is provided a use of any one ofthe complexes described herein, prepared by the process describedherein, as a hydrogenation pre-catalyst or hydrogenation catalyst tohydrogenate a substrate, wherein the substrate is a ketone, aldehyde, orimine.

In accordance with another embodiment, there is provided a use whereinthe hydrogenation pre-catalyst or hydrogenation catalyst is chiral andthe hydrogenation is an asymmetric hydrogenation.

In accordance with another aspect, there is provided a method forhydrogenation of a substrate comprising contacting the substrate with ahydrogen source in the presence of a complex described herein, underconditions suitable for hydrogenation.

In accordance with another embodiment, there is provided a method forhydrogenation wherein the substrate is a ketone, aldehyde, or imine, andthe hydrogen source is hydrogen gas at a pressure >0 atm and less than<70 atm.

BRIEF DESCRIPTION OF THE TABLES AND FIGURES

For a better understanding of the technology as described herein, aswell as other aspects and further features thereof, reference is made tothe following description which is to be used in conjunction with theaccompanying drawings and tables, where:

Table 1 delineates hydrogenation of acetophenone catalyzed by achiralcomplexes 4a-b once activated by reaction with LiAlH₄ and then alcohol;

Table 2 delineates reactivity of various ketones in an asymmetrichydrogenation reaction using an in situ generated catalyst derived from(S,S)-4d;

Table 2a delineates an asymmetric hydrogenation of acetophenone using anin situ generated chiral catalyst; and

Table 3 delineates enantio-determining step (EDS) transition stateenergies calculated using different functionals.

FIG. 1 depicts active iron catalysts for ketone (M1-3 and B1) and imine(B2) hydrogenation;

FIG. 2 depicts a template synthesis of iron complexes,trans-[Fe(P—CH═N—P′)(CO)₂(Br)][BF₄];

FIG. 3 depicts an ORTEP plot (thermal ellipsoids at 30% probability) ofthe X-ray crystal structure of trans-Fe(Cy₂PCH₂CH═NCH₂CH₂PPh₂)(CO)(Br)₂,2a. Hydrogen atoms of Ph and Cy substituents removed for clarity.Selected bond lengths (Å) and angles (deg): Fe(1)-P(1): 2.2680(9);Fe(1)-P(2): 2.2613(9); Fe(1)-N(1): 2.011(2); Fe(1)-Br(1): 2.4545(5);N(1)-C(2): 1.269(4); N(1)-C(3): 1.479(4): O(1)-C(5): 1.138(4);C(5)-Fe(1)-N(1): 177.6(1); P(2)-Fe(1)-P(1): 167.75(3);Br(1)-Fe(1)-Br(2): 175.12(2);

FIG. 4A depicts ORTEP an plot (thermal ellipsoids at 30% probability) ofthe X-ray crystal structure of trans-Fe(Cy₂PCH₂CH═NCH₂CH₂PPh₂)(CO)(Br)₂,2a;

FIG. 4B depicts an ORTEP plot (thermal ellipsoids at 30% probability) ofthe X-ray crystal structures of 4a;

FIG. 4C depicts ORTEP plot (thermal ellipsoids at 30% probability) ofthe X-ray crystal structures of 4b;

FIG. 4D depicts an ORTEP plot (thermal ellipsoids at 30% probability) ofthe X-ray crystal structure of cis-[Fe(P—N—P′)(CO)₂(Br)]₂[FeBr₄](cis-4a). Second cation of cis-[Fe(P—N—P′)(CO)₂(Br)]₂[FeBr₄], FeBr₄ ²⁻anion and hydrogen atoms of Ph and Cy substituents removed for clarity.Selected bond lengths (Å) and angles (deg): Fe(1)-Br(1): 2.444(2);Fe(1)-P(1): 2.281(3); Fe(1)-N(1): 2.030(7); Fe(1)-C(5): 1.77(1);Fe(1)-C(6): 1.82(1); O(1)-C(5): 1.13(1); N(1)-C(3): 1.29(1); N(1)-C(2):1.40(1); C(5)-Fe(1)-C(6): 90.0(5); P(2)-Fe(1)-P(1): 164.1(1);C(5)-Fe(1)-Br(1): 176.4(3); C(6)-Fe(1)-N(1): 178.9(4);

FIG. 5 depicts a ¹H NMR spectrum (600 MHz, THF-d8) ofhydride-aluminumhydrides of 5a and 6a. Note: a mono-hydride ironcompound was evident at −11.46 ppm (J_(HP)=64.1, 71.9 Hz);

FIG. 6 depicts observed and calculated hydride resonances of 6b. Left:hydride resonance at −11.18 ppm (²J_(HP)=28, 38 Hz; line width: 9.00).Right hydride resonance at −11.57 ppm (²J_(HP)=31, 37 Hz; line width:7.50). (Simulations were obtained from the MestReC NMR processingsoftware (version 4.7.0.0) using a Lorentzian distribution);

FIG. 7 depicts ¹H and ¹H{³¹P} NMR spectra (600 MHz, THF-d₈) of (S,S)-5d,6d and 6d (second diastereomer);

FIG. 8 depicts the structure of hydride-aluminumhydride complexFeH₂(CO)(PH₂CH₂CH₂N(AlH₂)CH₂CH₂PH₂) (compounds 5) calculated using DFT.Atom colours: iron, purple; phosphorus, orange; nitrogen, blue; oxygen,red; aluminum, pink; carbon gray; hydrogen, white.GAUSSIAN09/M06/6-31++G(d,p)/IEF-PCM (THF);

FIG. 9A depicts a structure of hydride-aluminumhydride complexLi[FeH₂(CO)(PH₂CH₂CH₂N(AlH₃)CH₂CH₂PH₂)] (compounds 6) calculated usingDFT. Atom colours: iron, purple; phosphorus, yellow; nitrogen, blue;oxygen, red; aluminum, pink; lithium, magenta; carbon gray; hydrogen,white. GAUSSIAN09/M06/6-31++G(d,p)/IEF-PCM (THF);

FIG. 9B depicts an ORTEP plot of complex 9 (thermal ellipsoids at 30%probability), wherein hydrogen atoms of phenyl and iso-propylsubstituents were removed for clarity;

FIG. 10A depicts simplified models of (S,S)-7d, wherein atoms C, H, N, Oand P were treated with a 6-31 G basis set;

FIG. 10B depicts relative energies of five possible diastereomers of(S,S)-7d. A sixth high energy diastereomer, D, with hydride trans to COwas not shown. Bond lengths for structure F: Fe—H=1.55, Fe—C=1.69,Fe—O=1.99, Fe—N=2.07, Fe—P (PPh2)=2.23, Fe—P (PCy₂)=2.24 Å, O . . .HN1.75 Å. Bond Angles: H—Fe—C=100.9°, H—Fe—P(PCy₂)=83.7°,H—Fe—P(PPh₂)=77.8°, H—Fe—N=89.6°, O—Fe—C=95.7°, O—Fe—P(PCy₂)=92.4°,O—Fe—P(PPh₂)=102.3°, O—Fe—N=73.7°, Fe—N—H=88.5°, Fe—O—H (N—H)=75.2°,N—H—O=119.7°, Fe—O—C=126.5°, Fe—C—O=173.7°, C—Fe—N=169.4°,P(PPh₂)-Fe—P(PCy₂)=158.9°, H—Fe—O=163.2°;

FIG. 11 depicts ¹H—³¹P HMBC spectrum (600 MHz, THF-d₈) of complexes 5cand 6c;

FIG. 12A depicts Gaussian09 M06/6-31++G(d,p)/(IEF-PCM)SMD(THF) resultsfor most stable simplified model of (S,S)-7d, F, with trans-(H)(OCH₃)and OCH₃ adjacent to N—H;

FIG. 12B depicts a GAUSSIAN09/M06/6-31++G(d,p)//IEF-PCM+SMD(THF)optimized structure, as well as selected bond lengths (Å) and angles(deg), of most stable isomer F, a simplified structure of (S,S)-7d;

FIG. 13 depicts optimized geometries, Gibbs free energy, and enthalpyfor 13a (S=0), 13a (S=1), 16a (S=0) and 16a (S=1), with hydrido amidocomplex 13a being relatively more stable in its diamagnetic form, andpossible Fe(0) product of reductive elimination of hydride and amide,Fe(CO)(P—NH—P′) 16a being relatively more stable in a high spin state;

FIG. 14 depicts previously known and herein described iron catalystswith tridentate P—N—P ligands;

FIG. 15 depicts activation of [Fe(PNP′)(CO)₂Br][BF₄] precatalyst withLiAlH₄ and alcohol;

FIG. 16 depicts proposed catalytic cycle with resting states andtransition states along with relative free energies (and enthalpies inbrackets, in kcal/mol) as calculated by DFT; relative energies of 7b and14a compared to 13a in absence of KOtBu indicated in bold;

FIG. 17 depicts selected geometries for species in catalytic cycle shownin FIG. 16; phenyl groups on one phosphorus were omitted for clarity;

FIG. 18 depicts a free energy profile of a catalytic pathway for complex11a;

FIG. 19 depicts free energy profile of a catalytic pathway using asimplified structure of (S,S)-11b, model complex A;

FIG. 20 depicts optimized geometries, as well as selected bond lengths(Å) and angles (deg.), of species P, TS_(P, Q) (382.85i cm⁻¹), Q,TS_(Q, R) (1252.40i cm⁻¹), R, T, TS_(T, U) (333.00i cm⁻¹), and U fromcatalytic cycle shown in FIG. 19, making use of simplified structure of(S,S)-11b where phenyls on phosphorus were replaced with hydrogens,phenyl on the backbone replaced by methyl, and isopropyls on phosphorusreplaced with methyls;

FIG. 21 depicts free energy (enthalpy) profile for alcohol-assistedhydrogen splitting in THF;

FIG. 22 depicts enantiodetermining step (EDS) geometries, top: Gaussviewmodel to illustrate relevant bond lengths, with phenyl and cyclohexylsubstituents removed for clarity; bottom: complete structures inspace-filling style; left: formation of S-isomer EDS_(S); right:formation of R-isomer EDS_(R); and

FIG. 23 depicts an energy profile for enantiodetermining steps (EDS) andrelevant ground states, with S-alcohol formation marked by ‘---’ andR-alcohol formation by ‘⋅⋅⋅⋅’.

DETAILED DESCRIPTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs.

As used in the specification and claims, the singular forms “a”, “an”and “the” include plural references unless the context clearly dictatesotherwise.

The term “comprising” as used herein will be understood to mean that thelist following is non-exhaustive and may or may not include any otheradditional suitable items, for example one or more further feature(s),component(s) and/or ingredient(s) as appropriate.

As used herein, “substituted” refers to the structure having one or moresubstituents. A substituent is an atom or group of bonded atoms that canbe considered to have replaced one or more hydrogen atoms attached to aparent molecular entity. Examples of substituents include aliphaticgroups, halogen, arylcarbonyloxy, alkoxycarbonyloxy, aryloxycarbonyloxy,alkoxycarbonyl, aminocarbonyl, alkylthiocarbonyl, alkoxyl, phosphateester, phosphonato, phosphinato, cyano, tertiary amino, tertiaryacylamino, tertiary amide, imino, alkylthio, arylthio, sulfonato,sulfamoyl, tertiary sulfonamido, nitrile, trifluoromethyl, heterocyclyl,aromatic, and heteroaromatic moieties, ether, ester, boron-containingmoieties, tertiary phosphines, and silicon-containing moieties. The term“optionally substituted” means unsubstituted or substituted.

As used herein, “alkyl” refers to a linear, branched or cyclic,saturated hydrocarbon group, which can be unsubstituted or is optionallysubstituted with one or more substituents. Examples of saturatedstraight or branched chain alkyl groups include, but are not limited to,methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl, 2-methyl-1-propyl,2-methyl-2-propyl, 1-pentyl, 2-pentyl, 3-pentyl, 2-methyl-1-butyl,3-methyl-1-butyl, 2-methyl-3-butyl, 2,2-dimethyl-1-propyl, 1-hexyl,2-hexyl, 3-hexyl, 2-methyl-1-pentyl, 3-methyl-1-pentyl,4-methyl-1-pentyl, 2-methyl-2-pentyl, 3-methyl-2-pentyl,4-methyl-2-pentyl, 2,2-dimethyl-1-butyl, 3,3-dimethyl-1-butyl and2-ethyl-1-butyl, 1-heptyl and 1-octyl. As used herein the term “alkyl”encompasses cyclic alkyls, or cycloalkyl groups.

The term “cycloalkyl” as used herein refers to a non-aromatic, saturatedmonocyclic, bicyclic or tricyclic hydrocarbon ring system containing atleast 3 carbon atoms. Examples of C₃-C₁₂ cycloalkyl groups include, butare not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl,cycloheptyl, cyclooctyl, norbornyl, adamantyl, bicyclo[2.2.2]oct-2-enyl,and bicyclo[2.2.2]octyl.

As used herein, the term “alkenyl” refers to a straight, branched orcyclic hydrocarbon group containing at least one double bond, which canbe unsubstituted or optionally substituted with one or moresubstituents.

As used herein, the term “alkoxy”, either alone or in combination withanother radical, refers to the radical —O—(C_(1-n))alkyl wherein thealkyl group contains one or more carbon atoms, and includes, forexample, methoxy, ethoxy, propoxy, 1-methylethoxy, butoxy,cyclohexyloxy, and 1,1-dimethylethoxy. “Alkoxide” refers to the radical—O—(C_(1-n))alkyl bearing a negative charge.

As used herein, “aryl” refers to hydrocarbons derived from benzene or abenzene derivative that are unsaturated aromatic carbocyclic groups offrom 6 to 100 carbon atoms, which may or may not be a fused ring system.In some embodiments the number of carbon atoms is from 6 to 50, in otherembodiments the number of carbon atoms is from 6 to 25, and in stillother embodiments the number of carbon atoms is from 6 to 15. An arylmay have a single ring or multiple rings. The term “aryl” as used hereinalso includes substituted aryls. Examples include, but are not limitedto phenyl, naphthyl, xylene, 1- or 2-phenylethyl, substituted phenyl,substituted naphthyl, substituted xylene, substituted 4-ethylphenyl andthe like.

As used herein, the term “aryloxy”, either alone or in combination withanother radical, refers to the radical —O-aryl, wherein aryl is asdefined above.

As used herein, “heteroaryl” refers to an aryl that includes from 1 to10, in other embodiments 1 to 4, heteroatoms selected from oxygen,nitrogen and sulfur, which can be substituted or unsubstituted. The term“heteroatom” refers to an atom that is not carbon or hydrogen, such asnitrogen, oxygen, sulfur, or phosphorus.

As used herein, “halogen” or “halo” refers to F, Cl, Br or I. The term“halide” refers to a halogen atom bearing a negative charge.

As used herein, a “coordinating atom” refers to an atom having a lonepair of electrons capable of coordinating, or forming a covalent dativebond, with a metal atom.

As used herein, a dashed line in a chemical structure is intended toindicate that a bond may or may not be present. In the case where twoadjacent bonds are shown with a dashed line to indicate the presence orabsence of a double bond, only one of the bonds can be a double bond. Itshould also be noted that any carbon as well as heteroatom with avalence level that appears to be unsatisfied as described or shownherein, or as a result of an optional bond being absent, is assumed tohave a sufficient number of hydrogen atom(s) to satisfy the valencesdescribed or shown.

As used herein, the term “electron withdrawing group” refers to anelectronegative group capable of polarizing a bond with a carbon atom.Some examples of electron withdrawing groups are halogens, CF₃, nitro,nitrile, carbonyl and substituted carbonyl.

The term “PNP” refers to the atoms in the ligand, in sequence, whichcoordinate to the metal centre of the catalyst. In the describedtridentate ligands, the coordinating atoms arephosphorus-nitrogen-phosphorous, hence P—N—P.

As used herein, the terms “catalyst”, “catalyst complex” and “complex”refer to the iron (II) species. These terms are used herein to refer toboth the catalyst precursors, as well as the active catalysts. Thesespecies can be neutrally charged and can exist without a counterion, orcan be positively charged and associated with one or morenon-coordinating anions(s) to balance the charge

As used herein, the term “non-coordinating counter-ion” refers to anegatively charged ion that associates with a positively chargedcatalyst; or, a positively charged ion that associates with thenegatively charged catalyst, to charge balance the catalyst complex. Thenon-coordinating anion can be any conjugate base of a strong acid, andthe non-coordinating cation can be an alkali metal.

As used herein, the term “ligand”, abbreviated L, refers to a chemicalspecies that coordinates with the iron centre of the catalyst. Theligand is a Lewis base that can be, for example: a carbon donor such ascarbon monoxide, carbene, cyanide or isocyanide (isonitrile); a nitrogendonor, such as nitrosyl, amine, imine, amide, N-heterocycles, nitriles,dinitrogen, or hydrazine; a phosphorous donor, such as phosphines orphosphites; a boron donor, such as boryl; a hydrogen donor, such asdihydrogen, hydride, borohydride, aluminum hydride or other hydridecomplexes; silane; a silicon donor, such as, silyl; an oxygen donor,such as alcohols, alkoxides, ethers, esters, amides, carboxylates,carboxylic acids, phosphine oxides, sulfoxides or sulfones; a sulfurdonor, such as thiols, sulfoxides, thiophenes or sulphides, a halidesuch as chloride, bromide, or iodide.

As used herein, the term “template synthesis” is method for synthesizinga ligand that is formed from precursor parts that coordinate to a metalion at geometrically defined positions, such as octahedral or squareplanar, and bond together. The metal ion acts as a template for theformation of this ligand. Given the same reaction conditions, but in theabsence of the metal template, the precursor parts usually do not react,or do react but form a mixture of products, none of which have thestructure of the ligand. The term “template ligand” refers to a ligandsynthesized via a “template synthesis”.

As used herein, the term “hydrogenation” refers to the movement,mediated by a catalyst, of gaseous dihydrogen to a molecule with anunsaturated group, such as a carbonyl or imine.

As used herein, the term “asymmetric hydrogenation” refers to ahydrogenation of a prochiral molecule, such as a ketone, or imine, toproduce an enantioenriched product, such as an alcohol or amine,catalyzed by an enantiomeric or enantiopure metal complex.

Overview

Described and disclosed herein are iron(II) complexes with PNP ligands,which are useful as catalytic materials for hydrogenation, and/orasymmetric hydrogenation of aldehydes, ketones, and/or imines.

The asymmetric hydrogenation method described herein, which produces aspecific enantiomer, enables a more economical, more efficient, safer,and greener chemical pathway to generate compounds that are enriched ina required enantiomer.

Conventional hydrogenation and asymmetric hydrogenation catalystsutilize platinum group metals (PGM) such as Ru, Os, Rh, Ir, Pd, and/orPt [De Vries et al., “Handbook of Homogeneous Hydrogenation” Wiley-VCH,volumes 1-3, 2007]. PGMs are expensive and, consequently, add to thecost of the final product. In addition, they are in limited supply andnot always readily available. By contrast, iron is relativelyinexpensive, abundant, and biocompatible. The iron(II) catalyst systems,containing tridentate diphosphine PNP ligands, described herein displaygood activity in hydrogenations, with good activity andenantioselectivity in asymmetric hydrogenations, of aldehydes, ketonesand/or imines.

Complexes

Described herein are catalysts useful for hydrogenation and/orasymmetric hydrogenation. Specifically, the described catalysts are iron(II) complexes containing tridentate diphosphine (PNP) ligands.

Complexes as described herein have the general structure of formula (I)

-   -   wherein:    -   a dashed line indicates that a bond may or may not be present;    -   each R¹ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₁-C₈        alkoxy, C₃-C₁₀ cycloalkyl, aryl, or heteroaryl, each of which        may be optionally substituted; or the two geminal R¹        substituents combine to form a C₂-C₄ linear alkyl diradical or        C₃-C₁₀ branched alkyl diradical, each of which may be optionally        substituted, such that the two R¹ substituents, together with        the phosphorus atom to which they are attached, form a ring;    -   each R² is independently aryl, heteroaryl, C₁-C₁₀ alkyl, C₂-C₁₀        alkenyl, C₁-C₁₀ alkoxy, aryloxy, or C₃-C₁₀ cycloalkyl, each of        which may be optionally substituted; or the two geminal R²        groups combine to form a C₂-C₄ linear alkyl diradical or C₃-C₁₀        branched alkyl diradical, each of which may be optionally        substituted, such that the two R² substituents, together with        the phosphorus atom to which they are attached, form a ring;    -   R³, R⁴, R⁵, R⁶, R⁸, R⁹, R¹⁰ and R¹¹ are each independently H, or        C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, or C₃-C₁₀        cycloalkyl, each of which may be optionally substituted; or, R³        and R⁴, R⁵ and R⁶, and/or R¹⁰ and R¹¹, together with the carbon        atoms to which they are attached, form a substituted C₅-C₁₀        cycloalkyl ring;    -   R⁷ is absent, H, AlH₃, or AlH₅;    -   each L¹ is independently H, BH₄, AlH₄, a halide, CO, an        N-heterocyclic carbene, OR¹², or NCR¹³, wherein R¹² and R¹³ are        independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, or        C₃-C₁₀ cycloalkyl, each of which may be substituted; or, one of        L¹ may be absent;    -   when R⁷ is AlH₃ or AlH₅, at least one of the H may bridge with        Fe to form a cycle together with the atoms to which they are        attached;    -   n is 0, +1, or −1, wherein, when n is +1, the complex further        comprises at least one non-coordinating anion, Y; and, when n is        −1, the complex further comprises at least one non-coordinating        cation, Z; such that the total charge of the complex is 0;    -   with the proviso that, when the nitrogen is singly bound to the        carbon attached to R⁹, each of R³ to R¹¹ are H, one L¹ is CO,        and the other two L¹'s are Br, or Br and H, or BH₄ and H, then        the R¹ and R² substituents cannot all be isopropyl; and    -   with the proviso that, when the nitrogen is singly bound to the        carbon attached to R⁹, each of R³ to R¹¹ are H, R⁷ is absent,        one L¹ is absent, one of L¹ is CO, and one of L¹ is H, then the        R¹ and R² substituents cannot all be isopropyl.

When the complex of formula (I) has L¹=H and OR¹², the complex will havethe structure of formula (Ia):

When the complex of formula (I) has R⁷=AlH₃ or AlH₅, the complex willhave the structure of formula (Ib) or (Ic):

When the complex of formula (I) has an imine at N and one L¹=a halide(X), the complex will have the structure of formula (Id):

When the complex of formula (I) has an imine at N and two L¹=a halide(X), the complex will have the structure of formula (Ie) or (If):

When the complex of formula (I) has two L¹=H, the complex will have thestructure of formula (Ih) or (Ig):

When the complex of formula (I) has one L¹=absent, and one L¹=H, thecomplex will have the structure of formula (Ii):

The complexes described herein can optionally comprise at least onenon-coordinating anion or cation. The non-coordinating anion can be anyconjugate case of a strong acid. Non-limiting examples of anions includehalides, BF₄ ⁻, PF₆ ⁻, SbF₆ ⁻, NO₃ ⁻, CF₃COO⁻, R¹⁴SO₃ ⁻, CF₃SO₃ ⁻,C₆H₅SO₃ ⁻, p-CH₃C₆H₄SO₃ ⁻, phosphates, TRISPHAT(Δ- or Λ-P(OC₆Cl₄O)₃ ⁻),carboranes, B(R¹⁴)₄ ⁻ or Al(R¹⁴)₄ ⁻, each of which may be substituted,wherein each R¹⁴ is independently an optionally substituted C₁-C₆ alkyl,aryl, heteroaryl, C₆H₃(CF₃)₂ and C₆F₅, halogen, pseudohalogen, C₁-C₈alkoxide, or aryloxide. In one embodiment, Y is BF₄ ⁻. Thenon-coordinating cation can be any alkali metal ion. Non-limitingexamples of cations include K⁺, Na⁺, or Li⁺.

The complexes described herein can optionally comprise chiral carbons,wherein the complexes are enantiomerically enriched, or racemates.Non-limiting examples of chiral complexes are as follows:

Synthesis of Iron Complexes

Previously reported multi-component template syntheses of iron(II)complexes (patent application U.S. Ser. No. 12/609,955 andPCT/CA2013-050405, each of which is incorporated herein by reference inits entirety) were modified and implemented in the synthesis of theherein described complexes.

Consequently, there is provided herein a process for preparing any oneof the complexes (I), and (Ia)-(If) herein described, the processcomprising reacting a phosphine-aldehyde precursor of formula (II)

wherein R¹, R⁸, R¹⁰, and R¹¹ are as defined above,

-   with a phosphine-amine of formula (III)

wherein R², R³, R⁴, R⁵, and R⁶ are as defined above,

-   in the presence of-   an iron(II) compound,-   a CO atmosphere, and-   a strong base,-   to form a complex of formula (Ie), or to form a mixture of complexes    of formula (Ie) and formula (If),

wherein X is a halide, and

-   L¹ is CO;-   the complex of formula (Ie), or the mixture of complexes of formula    (Ie) and formula (If), is further reacted in the presence of-   a silver salt, and-   a CO atmosphere,-   to form a complex of formula (1d)

which is further reacted in the presence of a reducing agent to form thecomplex of formula (Ib) and/or (Ic)

wherein R⁹ is H,

-   which are further reacted with an excess of a primary, secondary, or    tertiary alcohol, to form the complex of formula (Ia)

The process described herein can optionally involve phosphine-amines offormula (III), and/or phosphine-aldehyde precursors of formula (II) thatare chiral, wherein each precursor is optionally enantiomericallyenriched, or a racemate.

In one embodiment of the process described herein, the iron source is aniron(II) salt such as FeBr₂ or FeCl₂; or, an iron(II) complex such asFe(CO)₄Br₂, which is additionally reacted in the presence of UVradiation to aid in formation of the complex of formula (Ie); or, toform a mixture of complexes of formula (Ie) and formula (If). In otherembodiments, the iron source can be a Fe(II) salt with halide(s) orpseudo halide(s) that are soluble in common organic solvents, with thehalide or pseudo halide being capable of precipitating as an insolublesilver salt or sodium salt in preparation of complexes (I) (e.g.complexes 4a-4d below).

In one embodiment, the silver salt is AgBF₄, the reducing agent isLiAlH₄ or NaAlH₄, and the alcohol is MeOH, EtOH, tBuOH, or ^(t)AmylOH.

There is also provided herein a process for preparing any one of thecomplexes (Ig)-(Ig) herein described, the process comprising reacting acomplex of formula (Ia)

-   -   wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R¹⁰, R¹¹, R¹² and L¹ are        as defined above,    -   in the presence of    -   a base, and    -   a H₂ atmosphere    -   to form a complex of formula (Ig) and/or a complex of formula        (Ih),

In one embodiment of the process described herein, carbon C¹, C² or C³,or any combination thereof, of complexes of formula (Ia), (1g), or (1h)is chiral, and the complex is enantiomerically enriched, or a racemate.In another embodiment, the base is KOtBu, NaOtBu, Ph-CH(OK)CH₃, orNaOMe.

Catalytic Hydrogenation Methods

The herein described iron(II) complexes are useful for catalytichydrogenation of unsaturated chemical bonds such as, for example, iminesand carbonyl groups. These complexes are also useful for the effectiveasymmetric hydrogenation of substrates, such as ketones and imines togive enantiomeric or enantiomerically enriched products, such asalcohols and amines. In one embodiment, the activated iron(II) complexeshave been shown to be capable of transferring gaseous H₂ to a carbonylor imine polar bond.

As such, described herein is a use of any one of the herein describedcomplexes, prepared by the herein described process, as a hydrogenationpre-catalyst or hydrogenation catalyst to hydrogenate a substrate, suchas a ketone, aldehyde, or imine.

In one embodiment of this use, the complexes are chiral and thehydrogenation is an asymmetric hydrogenation.

The herein described catalysts can be utilized for preparing alcohols.Imine groups can similarly be hydrogenated or asymmetricallyhydrogenated to provide amines. It is understood that when anenantiopure catalyst is used the products of these organic reactions canbe enantioenriched when a reactant is prochiral.

The products of the herein described hydrogenation reactions may beuseful in subsequent reactions to prepare commercial end products, suchas, for example, pharmaceuticals, agrichemicals, cosmetics andnutriceuticals.

As such, described herein is a method for hydrogenation of a substratecomprising contacting the substrate with a hydrogen source in thepresence of any one of the herein described complexes, under conditionssuitable for hydrogenation.

In one embodiment of this method, the substrate is a ketone, aldehyde,or imine. In another embodiment, the hydrogen source is hydrogen gas ata pressure >0 atm and less than <70 atm.

To gain a better understanding of the invention described herein, thefollowing examples are set forth. It should be understood that theseexamples are for illustrative purposes only. Therefore, they should notlimit the scope of this invention in any way.

EXAMPLES

General Methods

All procedures and manipulations involving air-sensitive materials wereperformed under an argon or nitrogen atmosphere using Schlenk techniquesor a glove-box with N₂(g). Solvents were degassed and dried usingstandard procedures prior to all manipulations and reactions. Deuteratedsolvents were purchased from Cambridge Isotope Laboratories orSigma-Aldrich, degassed and dried over activated molecular sieves priorto use. All liquid ketone substrates were vacuum distilled, degassed andstored over activated molecular sieves. Phosphonium dimers 1a-c [seeFIG. 2; Lagaditis, P. O.; Mikhailine, A. A.; Lough, A. J.; Morris, R. H.Inorg. Chem. 2010, 49, 1094-1102; Mikhailine, A. A.; Lagaditis, P. O.;Sues, P. E.; Lough, A. J.; Morris, R. H. J. Organomet. Chem. 2010, 695,1824-1830], and Fe(CO)₄(Br)₂ [Turrell, P. J.; Wright, J. A.; Peck, J. N.T.; Oganesyan, V. S.; Pickett, C. J. Angew. Chem. Int. Ed. 2010, 49,7508-7511] were synthesized according to literature procedures.2-(diphenylphosphino) ethylamine was donated by Digital SpecialtyChemicals. All other reagents were purchased from Sigma-Aldrich or StremChemicals and utilized without further purifications. NMR spectra wererecorded at ambient temperature and pressure using Varian Gemini [¹H(400, 600 MHz), ¹³C{¹H} (100, 150 MHz), ³¹P{¹H} (161, 242 MHz), ¹⁹F{¹H}(356 MHz)]. ³¹P chemical shifts were measured relative to 85% H₃PO₄ asan external reference. ¹⁹F chemical shifts were referenced relative toCFCl₃. In the synthesis of 2a and b (see FIG. 2), photolysis wasperformed using a 450 W mercury vapor lamp (model: Hanovia UV MediumPressure 450 W Immersion Lamp). The elemental analyses were performed ona Perkin-Elmer 2400 CHN elemental analyzer. Some complexes gaveunsatisfactory carbon analyses but acceptable hydrogen and nitrogencontent because of a combustion problem due to the tetrafluoroborateanion [Marcó, A.; Compano, R.; Rubio, R.; Casals, I. Microchimica Acta2003, 142, 13-19].

Example 1 Template Synthesis of Iron(II) Complexes Experimental

trans-Fe(Ph₂PCH₂CH₂NCHCH₂PCy₂)(CO)(Br)₂, 2a: A vial was charged with 1a(80 mg, 0.125 mmol) and KOtBu (28 mg, 0.249 mmol) and 25 mL benzene,forming a slurry. The slurry was allowed to stir for 10 min by whichtime it turned cloudy and was then filtered into a Schlenk flask. Tothis solution, 2-(diphenylphosphino)ethylamine (57 mg, 0.249 mmol) wasadded, followed by Fe(CO)₄(Br)₂ (81 mg, 0.249 mmol). The mixtureimmediately evolved gas in addition to turning orange in color. Theflask was immediately exposed to UV light and was allowed to stir for 6hours or until the solution turned red-purple. The flask was removedfrom the UV source and filtered through a pad Celite to remove allprecipitates. The solvent was concentrated and pentane (10 mL) was addedto cause precipitation of a pale pink solid. The solid was washed withpentane (5 mL) and dried under vacuum. Yield: 35% (60 mg). ¹H NMR (400MHz, C₆D₆) δ: 1.18-1.95 (m, HCy), 2.47 (m, 2H, CH₂PCy₂), 2.66 (m, 2H,CHCy), 2.79 (m, 2H, CH₂PPh₂), 4.11 (m, 2H, CH₂N), 6.94 (m, 2H, HPh),7.05 (m, 4H, HPh), 7.91 (m, 5H, HPh, CHN). ¹³C {¹H} NMR (100 MHz, C₆D₆)δ: 26.5 (CCy), 27.9 (CCy), 28.7 (CH₂PPh₂), 29.9 (CCy), 30.6 (CH₂PCy₂),36.4 (CCy), 62.6 (CH₂N), 129.5 (CPh), 133.1 (CPh), 135.7 (CPh), 146.6(CPh), 173.3 (CHN), 227.8 (br t, J_(PC)=24.0 Hz, CO). ³¹P {¹H} NMR (161MHz, C₆D₆) δ: 68.2 (d, J_(PP)=174 Hz), 71.2 (d, J_(PP)=174 Hz) ppm. IR(KBr) 1945 cm⁻¹ (ν_(C≡O)). Anal. Calcd for C₂₉H₃₉NOP₂FeBr₂: C, 50.10; H,5.65; N, 2.01. Found: C, 49.52; H, 5.88; N, 1.75. MS (ESI,methanol/water; m/z⁺): 586.1 [C₂₈H₃₉NP₂FeBr]⁺. FIG. 4A depicts An ORTEPplot (thermal ellipsoids at 30% probability) of the X-ray crystalstructure of trans-Fe(Cy₂PCH₂CH═NCH₂CH₂PPh₂)(CO)(Br)₂, 2a, whereinhydrogen atoms of Ph and Cy substituents removed for clarity. Selectedbond lengths (Å) and angles (deg): Fe(1)-P(1): 2.2680(9); Fe(1)-P(2):2.2613(9); Fe(1)-N(1): 2.011(2); Fe(1)-Br(1): 2.4545(5); N(1)-C(2):1.269(4); N(1)-C(3): 1.479(4): O(1)-C(5): 1.138(4); C(5)-Fe(1)-N(1):177.6(1); P(2)-Fe(1)-P(1): 167.75(3); Br(1)-Fe(1)-Br(2): 175.12(2).

trans-Fe(Ph₂PCH₂CH₂NCHCH₂PiPr₂)(CO)(Br)₂, 2b: The pale pink solidproduct was synthesized and isolated using the procedure outlined fortrans-[Fe(Ph₂PCH₂CH₂NCHCH₂PCy₂)(CO)(Br)₂]: 1b (80 mg, 0.166 mmol); KOtBu(38 mg, 0.331 mmol); 2-(diphenylphosphino) ethylamine (76 mg, 0.331mmol); Fe(CO)₄(Br)₂ (108 mg, 0.331 mmol). Yield: 25% (50 mg). ¹H NMR(400 MHz, C₆D₆) δ: 0.90 (dd, J_(HP)=7, 14 Hz, 1H, CH(CH₃)₂), 1.21 (dd,J_(HP)=7, 13 Hz, 6H, CH₃), 1.44 (dd, J_(HP)=7, 13 Hz, 6H, CH₃), 1.70(dd, J_(HP)=7, 16 Hz, 1H, CH(CH₃)₂), 2.74 (m, 2H, CH₂PiPr₂) 2.77 (m, 2H,CH₂PPh₂), 4.13 (m, J_(HP)=20.6 Hz, 2H, CH₂N), 6.93 (m, 4H, HPh), 7.04(m, 2H, HPh), 7.22 (indirectly determined from ¹H-¹³C HSQC, CHN), 7.89(m, 2H, HPh), 7.98 (m, 2H, HPh), 8.45 (m, 1H, HPh). ¹³C {¹H} NMR (100MHz, C₆D₆) δ: 19.35 (CH₃), 19.5 (CH(CH₃)₂), 20.5 (CH₃), 25.8 (CH(CH₃)₂),28.2 (d, J_(CP)=20 Hz, CH₂PPh₂), 38.0 (d, J_(CP)=17 Hz, CH₂PiPr₂), 61.3(CH₂N), 129.4 (CPh), 131.5 (CPh), 132.8 (CPh), 135.3 (CPh), 171.8 (CHN),227.9 (t, J_(CP)=23.8 Hz, CO). ³¹P {¹H} NMR (161 MHz, C₆D₆) δ: 67.4 (d,J_(PP)=176 Hz), 79.7 (d, J_(PP)=176 Hz) ppm. IR (KBr) 1945 cm⁻¹(ν_(C≡O)). Anal. Calcd for C₂₃H₃₁NOP₂FeBr₂: C, 44.91; H, 5.08; N, 2.28.Found: C, 43.99; H, 5.12; N, 2.30. MS (ESI, methanol/water; m/z⁺): 507.2[C₂₂H₃₁NP₂FeBr]⁺.

³¹P{¹H}NMR Chemical Shifts (ppm) and Coupling Constants (Hz) ofCompounds 2 and 3^(a):

a 67.8 and 71.9 39.6 and 60.5 R = Cy, X = Br ²J_(PP) = 175.0 ²J_(PP) =201.3 b 67.36 and 79.9 40.5 and 70.5 R = iPr, X = Br ²J_(PP) = 176.0²J_(PP) = 207.5 c 70.0 and 74.0 42.8 and 49.6 R = Ph, X = Br ²J_(PP) =192.0 ²J_(PP) = 222.8 b-Cl 63.8 and 78.7 40.3 and 71.9 R = iPr, X = Cl²J_(PP) = 183.2 ²J_(PP) 217.6 ^(a) Assignment of cis and trans isomersc.f. the isolation of trans-Fe(P_(Cy)—CH═N—P_(Ph))(Br)₂(CO).

trans-[Fe(Ph₂PCH₂CH₂NCHCH₂PCy₂)(CO)₂(Br)][BF₄], 4a: A Schlenk flask wascharged with 1a (200 mg, 0.311 mmol) and KOtBu (70 mg, 0.623 mmol) and25 mL THF, forming a slurry. The slurry was allowed to stir for 10 minby which time the mixture turned cloudy. To this solution,2-(diphenylphosphino)ethylamine (143 mg, 0.623 mmol) was added, followedby FeBr₂ (204 mg, 0.934 mmol). The Schlenk flask was then exposed to anatmosphere of CO (˜2 atm); upon exposure to CO, the pale yellow slurryimmediately turned dark purple. The reaction mixture was allowed to stirfor 5 hours by which time it was red-purple in color. The solvent wasremoved and any residue was taken up in 25 mL dichloromethane. Thissolution was filtered through a pad of Celite into a new Schlenk flaskand exposed again to a CO atm. AgBF₄ (130 mg, 0.668 mmol) in 5 mL THFwas injected into the reaction mixture. The solution immediately changedto a bright purple color. After stirring for 30 min, the solvent wasremoved, taken up in dichloromethane and filtered through a pad ofCelite to remove a grey precipitate. The solvent was concentrated andpentane (10 mL) was added to cause precipitation of a purple solid. Thesolid was washed with pentane (5 mL) and dried under vacuum. Yield: 89%(454 mg). ¹H NMR (400 MHz, CD₂Cl₂) δ: 1.16-2.50 (m, 22H, HCy), 2.50 (m,2H, CH₂PCy₂), 2.92 (m, 2H, CH₂PPh₂), 3.63 (m, 2H, CH₂N), 7.56-7.95 (m,11H, HPh, HCN). ¹³C {¹H} NMR (100 MHz, CD₂Cl₂) δ: 25.7 (CCy), 27.2(CCy), 28.5 (CCy), 28.8 (CCy), 37.5 (CH₂PPh₂), 37.7 (CH₂PCy₂), 63.6(CH₂N), 129.4 (CPh), 131.0 (CPh), 131.8 (CPh), 182.0 (HCN), 211.5 (dd,J_(CP)=22, 25 Hz, CO). ³¹P {¹H} NMR (161 MHz, CD₂Cl₂) δ: 45.7 (d,J_(PP)=85 Hz, PPh₂), 70.8 (d, J_(PP)=85 Hz, PCy₂). ¹⁹F{¹H} NMR (356 MHz,CD₂Cl₂) δ: −155.5 (s, BF₄) ppm. IR (KBr) 2005 cm⁻¹ (ν_(C≡O)). Anal.Calcd for C₃₀H₃₉NO₂P₂FeBrBF₄: C, 49.35; H, 5.38; N, 1.92. Found: C,45.94; H, 5.47; N, 1.52. MS (ESI, methanol/water; m/z⁺): −644.1[C₃₀H₃₉NO₂P₂FeBr]⁺. FIG. 4B depicts an ORTEP plot (thermal ellipsoids at30% probability) of the X-ray crystal structures of 4a, where BPh₄ anionand hydrogen atoms of substituents removed for clarity. Selected bondlengths (Å) and angles (deg) of 4a: Fe(1)-C(6): 1.829(4); Fe(1)-N(1):1.980(3); Fe(1)-P(2): 2.271(1); Fe(1)-Br(1): 2.4416(6); O(1)-C(5):1.104(4); O(2)-C(6): 1.132(4); N(1)- C(3): 1.275(5); N(1)-C(2):1.482(4); C(6)-Fe(1)-C(5): 172.2(2); P(2)-Fe(1)-P(1): 168.38(4);N(1)-Fe(1)-Br(1): 175.35(9).

trans-[Fe(Ph₂PCH₂CH₂NCHCH₂PiPr₂)(CO)₂(Br)][BF₄], 4b: The purplesolidproduct was synthesized and isolated using the procedure outlinedfor 4a: 1b (200 mg, 0.415 mmol); KOtBu (93 mg, 0.830 mmol);2-(diphenylphosphino)ethylamine (190 mg, 0.830 mmol); FeBr₂ (271 mg,1.245 mmol); AgBF₄ (201 mg, 1.035 mmol). Yield: 83% (450 mg). ¹H NMR(400 MHz, CD₂Cl₂) δ: 1.13 (m, 2H, CH(CH₃)₂), 1.45 (m, 12H, CH₃), 2.78(m, 2H, CH₂PiPr₂), 2.90 (m, 2H, CH₂PPh₂), 3.62 (m, 2H, CH₂N), 7.53 (m,4H, HPh), 7.91 (m, 7H, HPh, CHN). ¹³C {¹H} NMR (100 MHz, CD₂Cl₂) δ: 15.5(CH(CH₃)₂), 19.1 (CH₃), 28.2 (CH₂PiPr₂), 28.3 (CH₂PPh₂), 63.0 (CH₂N),129.8 (CPh), 131.8 (CPh), 132.1 (CPh), 181.7 (HCN), 211.5 (dd,J_(CP)=22, 25 Hz, CO). ³¹P {¹H} NMR (161 MHz, CD₂Cl₂) δ: 45.5 (d,J_(PP)=86 Hz, PPh₂), 78.2 (d, J_(PP)=86 Hz, PiPr₂). ¹⁹F{¹H} NMR (356MHz, CD₂Cl₂) δ: −155.5 (s, BF₄) ppm. IR (KBr) 2011 cm⁻¹ (ν_(C≡O)). Anal.Calcd for C₂₄H₃₁NO₂P₂FeBr₂BF₄: C, 44.35; H, 4.81; N, 2.15. Found: C,43.21; H, 4.88; N, 2.09. MS (ESI, methanol/water; m/z⁺): 563.2[C₂₄H₃₁NP₂O₂FeBr]⁺. FIG. 4C depicts an ORTEP plot (thermal ellipsoids at30% probability) of the X-ray crystal structures of 4b. Selected bondlengths (Å) and angles (deg) of 4b: Fe(1)-P(1): 2.265(2); Fe(1)-N(1):1.980(4); Fe(1)-C(12): 1.803(6); Br(1)-Fe(1): 2.4530(8); O(2)-C(12):1.144(6); N(1)-C(2): 1.270(6); N(1)-C(3): 1.477(7); C(12)-Fe(1)-C(11):170.4(2); P(1)-Fe(1)-P(2): 167.40(5); N(1)-Fe(1)-Br(1): 174.9(1).

trans-[Fe(Ph₂PCH₂CH₂NCHCH₂PPh₂)(CO)₂(Br)][BF₄], 4c. The purple solidproduct was synthesized and isolated using the procedure outlined for4a: 1c (200 mg, 0.324 mmol); KOtBu (73 mg, 0.648 mmol);2-(diphenylphosphino)ethylamine (149 mg, 0.648 mmol); FeBr₂ (208 mg,0.971 mmol); AgBF₄ (157 mg, 0.809 mmol). Yield: 89% (454 mg). ¹H NMR(400 MHz, CD₂Cl₂) δ: 2.16 (m, 2H, CH₂PPh₂), 2.99 (m, 2H, CH₂N), 3.77 (m,2H, CH₂PPh₂), 7.54-7.94 (m, 21H, HPh, CHN). ¹³C {¹H} NMR (100 MHz,CD₂Cl₂) δ: 29.2 (CH₂PPh₂), 31.7 (CH₂Ph₂), 62.5 (CH₂N), 129.4 (CPh),131.6-132.2 (CPh), 182.4 (CHN), 208.8 (br t, J_(CP)=24 Hz, CO). ³¹P {¹H}NMR (161 MHz, CD₂Cl₂) δ: 47.4 (d, J_(PP)=95 Hz, PPh₂), 51.9 (d,J_(PP)=95 Hz, PCy₂). ¹⁹F{¹H} NMR (356 MHz, CD₂Cl₂) δ: −155.5 (s, BF₄)ppm. IR (KBr) 2016 cm⁻¹ (ν_(C≡O)). Anal. Calcd for C₃₀H₂₇NO₂P₂FeBrBF₄:C, 50.18; H, 3.79; N, 1.92. Found: C, 44.38; H, 4.21; N, 1.58. MS (ESI,methanol/water; m/z⁺): 632.0 [C₃₀H₂₇NO₂P₂FeBr]⁺.

trans-(S,S)-[Fe(Ph₂PCH(Ph)CH(Me)NCHCH₂PCy₂)(CO)₂(Br)][BF₄], (S,S)-4d:

The red-purple solid product was synthesized and isolated using theprocedure outlined for 4a: 1a (100 mg, 0.156 mmol); KOtBu (35 mg, 0.312mmol); (S,S)-Ph₂PCH(Ph)CH(Me)NH₂ (100 mg, 0.312 mmol); FeBr₂ (100 mg,0.312 mmol); AgBF₄ (70 mg, 0.359 mmol). Yield: 82% (200 mg). ¹H NMR (400MHz, THF-d₈) δ: 0.72-2.05 (m, HCy), 1.11 (CH₃, indirectly determined via¹H—¹H COSY), 2.21 (m, 1H, HCy), 2.61 (m, 1H, HCy), 3.43 (CH₂PCy₂,indirectly determined via ¹H—¹H COSY), 3.67 (CH₂PCy₂, indirectlydetermined via ¹H—¹H COSY), 3.85 (m, 1H, CH(Me)), 4.14 (m, 1H, CH(Ph)),6.80-7.97 (m, 16H, HPh), 7.96 (d, indirectly determined via ¹H—¹H COSY,J_(HP)=20 Hz, CHN). ¹³C {¹H} NMR (100 MHz, THF-d₈) δ: 12.5 (CCy), 17.2(CCy), 21.3 (CCy), 27.1 (CCy), 28.4 (CCy), 35.4 (CH₂PCy₂), 37.3 (CCy),38.2 (CCy), 51.8 (CH(Ph)), 70.4 (CH(Me)), 126.0-135.3 (CPh), 179.3(CHN), 210.5 (br t, J_(CP)=23.5 Hz, CO), 214.7 (br t, J_(CP)=21.3 Hz,CO). ³¹P {¹H} NMR (161 MHz, THF-d) δ: 69.2 (d, J_(PP)=81 Hz), 67.8 (d,J_(PP)=81 Hz). ¹⁹F{¹H}NMR (356 MHz, THF-d₈) □: −155.9 (s, BF₄) ppm. IR(KBr) 2000.0 cm⁻¹ (ν_(C≡O)). Anal. Calcd for C₃₇H₄₅NO₂P₂FeBrBF₄: C,54.17; H, 5.53; N, 1.71. Found: C, 46.31; H, 6.08; N, 1.05. MS (ESI,methanol/water; m/z⁺): 734.1 [C₃₇H₄₅NO₂P₂FeBr]⁺.

cis-[Fe(Ph₂PC₂H₄NCHCH₂PCy₂)(CO)₂(Br)][BPh₄] (cis-4a):¹H NMR (400 MHz,CD₂Cl₂) δ: 1.31-2.11 (m, 22H, HCy), 2.61 (m, 2H, CH₂PPh₂), 2.79 (m, 2H,CH₂PCy₂), 3.00 (m, 1H, CH₂N), 3.44 (ddd, J_(HH)=12.0, 8.0 Hz,J_(PH)=40.0 Hz, CH₂N), 6.15 (d, J_(HP)=25.0 Hz, HCN), 6.86-7.70 (m, 30H,HPh). ¹³C {¹H} NMR (100 MHz, CD₂Cl₂) δ: 25.46 (d, J_(CP)=13.6, CCy),26.20 (CCy), 26.50 (CCy), 25.52 (d, J_(CP)=10 Hz, CCy), 26.69 (d,J_(CP)=11.6 Hz, CCy), 27.12 (d, J_(CP)=10.4 Hz, CCy), 27.48 (d,J_(CP)=12.8 Hz, CCy), 27.97 (CCy), 28.65 (CCy), 29.95 (d, J_(CP)=5.1 Hz,CCy), 30.50 (CCy), 36.80 (d, J_(CP)=24.0 Hz, CH₂PPh₂), 37.2 (d,J_(CP)=25.0 Hz, CH₂PCy₂), 38.16 (d, J_(CP)=18.4 Hz, CCy), 61.84 (CH₂N),122.40 (m, BCPh), 126.30 (m, BCPh), 128.90 (d, J_(CP)=9.0 Hz, CPh),129.80 (d, J_(CP)=10.0 Hz, CPh), 130.21 (d, J_(CP)=9.0 Hz, CPh), 131.20(d, J_(CP)=9.0 Hz, CPh), 132.70 (d, J_(CP)=8.0 Hz, CPh), 136.30 (m,BCPh), 164.30 (m, J_(CB)=49.0 Hz, BPh), 179.50 (HCN), 210.70 (t,J_(CP)=21.1 Hz, CO), 211.80 (br. dd, J_(CP)=19.0 Hz, CO). ³¹P {¹H} NMR(161 MHz, CD₂Cl₂) δ: 59.6 (d, J_(PP)=145 Hz, PPh₂), 78.9 (d, J_(PP)=145Hz, PCy₂) ppm. IR (KBr) 2038 cm⁻¹ (ν_(C≡O)); 1990 cm⁻¹ (ν_(C≡O)). Anal.Calcd for C₅₄H₅₉NO₂P₂FeBrB: C, 67.38; H, 6.18; N, 1.46. Found: C, 65.84;H, 6.56; N, 1.49.

General Synthesis of Mer-trans-[Fe(Br)(CO)₂(P—N—P′)][BF₄] Precatalysts(S)-4e,f,g (Table 2a):

In a nitrogen filled glovebox, dicyclohexylphosphonium dimer (0.05 g,0.078 mmol) and potassium tert-butoxide (0.018 g, 0.16 mmol) werestirred in 8 mL THF for 10 minutes to yield a cloudy white solution. Tothis solution, a phosphine-amine of formula (III) (0.16 mmol) and FeBr₂(0.05 g, 0.23 mmol) were added, yielding a pale yellow solution, and theflask was transferred to a Schlenk line and put under a CO_((g))atmosphere. Immediately upon exposure, the solution turned purple. Thissolution was stirred under CO_((g)) (˜2 atm) for 5 hours to yield a deepred-purple solution, which was then dried under reduced pressure,transferred to a nitrogen filled glovebox, and redissolved in 8 mL ofDCM. This solution was filtered through Celite, transferred back to aSchlenk line, and exposed to a CO_((g)) atmosphere. AgBF₄ (0.033 g, 17mmol) in 2 mL of THF was injected into the solution and stirred for 30minutes. Solvent was removed under reduced pressure to give a residue,and the flask transferred back into a nitrogen filled glovebox. Theresidue was redissolved in 5 mL of DCM, filtered through Celite, andconcentrated to ˜1 mL under reduced pressure. 5 mL of pentane was addedto precipitate out a deep purple powdered product, which was washed withdiethyl ether, and dried under reduced pressure.

(S)-4e: R=Cy, R¹=iPr, R²=H (Used AgPF₆ instead of AgBF₄); Yield=120mg=78%; ¹H NMR (400 MHz, THF-d₈) δ: 8.16 (m, 1H, Ph-CH), 7.92 (m, 1H,Ph-CH), 7.05-7.68 (m, 9H, Ph-CH and N═CH at 7.58—determined indirectlyfor ¹H-¹³C HSQC), 3.26 (m, 1H, N—C(iPr)H), 2.98 (m, 2H, CH₂—PPh), 2.35(m, 2H, CH₂—PCy), 1.21 (iPr-CH, determined indirectly for ¹H—¹H COSY),0.75 (m, 6H, iPr-CH₃) and 0.6-2.1 (m, PCy-H) ppm; ³¹P {¹H} (161 MHz,THF-d₈) δ: 63.28 (d, ²J_(PP)=81.6 Hz), 46.26 (d, ²J_(PP)=81.6 Hz) and−137.56 (m, PF₆) ppm; ¹³C {¹H} (100 MHz, THF-d₈) δ: 163.19 (N═CH),129-135 (Ph-CH), 67.02 (N—C(iPr)H), 42.70 (CH₂—PPh), 35.80 (PCy₂-C),32.52 (CH₂—PCy), 20-30 (PCy₂-C), 24.07 (iPr-CH) and 15.23 (iPr-CH₃) ppm;¹⁹F {¹H} (356 MHz, THF-d₈) δ: −64.13 (d, PF₆ ⁻, J=790 Hz) ppm; Anal.Calcd. for [FeC₃₇H₄₅P₂NO₂Br][PF₆]: C, 47.73; H, 5.46; N, 1.69. Found: C,40.09; H, 6.17; N, 1.80. ** MS (ESI, m/z⁺): 686.1 [FeC₃₃H₄₅P₂NO₂Br]⁺ and628.2 [FeC₃₁H₄₅P₂NBr]⁺ (loss of two —CO). IR: v(CO)=2005.8 cm⁻¹.

(S)-4f: R Cy, R¹=Ph, R²=H; Yield=110 mg=86%; ¹H NMR (400 MHz, THF-d₈) δ:8.15 (m, 2H, Ar—CH and P—Ar—CH), 8.01 (m, 2H, Ar—CH and P—Ar—CH),7.1-7.6 (m, Ar—CH and P—Ar—CH), 7.21 (N═CH, determined indirectly from¹H—¹H COSY), 4.51 (t, 1H, N—CH, J=11.6 Hz), 3.63 (CH₂—PPh₂, determinedindirectly from ¹H—¹H COSY), 3.58 (CH₂—PCy₂, determined indirectly from¹H—¹H COSY), 3.39 (CH₂—PCy₂, determined indirectly from ¹H—¹H COSY),3.13 (dd, 1H, CH₂—PPh₂, J=5.1 and 13.1 Hz) and 0.9-2.6 (m, PCy-H) ppm;³¹P {¹H} (161 MHz, THF-d₈) δ: 66.76 (d, ²J_(PP)=81.9 Hz) and 39.35 (d,²J_(PP)=81.6 Hz) ppm; ¹³C {¹H} (100 MHz, THF-d) b: 181.6 (N═CH), 128-133(Ar—CH and P—Ar—CH), 74.1 (N—CH), 38.5 (PCy-C), 36.1 (PCy₂-CH₂), 34.5(PPh₂-CH₂), 25-29 (PCy-C) and 13.3 (PCy₂-C) ppm; ¹⁹F {¹H} (356 MHz,THF-d₈) δ: −153 ppm; Anal. Calcd. for [FeC₃₆H₄₃P₂NO₂Br][BF₄]: C, 53.6;H, 5.40; N, 1.70. Found: C, 41.93; H, 4.98; N, 1.40**; MS (ESI, m/z⁺):720.1 [FeC₃₆H₄₃P₂NO₂Br]⁺. IR: v(CO)=2009.2 cm⁻¹.

(S)-4g: R¹=CH₂Ph, R²=H; Yield=110 mg=84%; ¹H NMR (400 MHz, THF-d₈) b:8.13 (m, 1H, N═CH), 7.74 (m, 1H, Ar—CH and P—Ar—CH), 6.9-7.5 (m, 14H,Ar—CH and P—Ar—CH), 3.61 (N—CH, determined indirectly from ¹H—¹H COSY),3.30 (CH₂—PCy₂, determined indirectly from ¹H—¹H COSY), 3.03 (CH₂—PPh₂,determined indirectly from ¹H—¹H COSY), 2.86 ((CH₂—PPh₂, determinedindirectly from ¹H—¹H COSY), 1.33 (CH₂-Ph, determined indirectly from¹H—¹H COSY) and 0.8-2.5 (m, PCy-H) ppm; ³¹P {¹H} (161 MHz, THF-d₈) δ:64.20 (d, ²J_(PP)=82.1 Hz) and 42.55 (d, ²J_(PP)=82.1 Hz) ppm; ¹³C {¹H}(100 MHz, THF-d₈) δ: 146.7 (N═CH), 127-133 (Ar—CH and P—Ar—CH), 66.5(N—CH), 41.1 (CH₂—PPh₂), 38.6 (CH₂—PCy₂), 26.9 (CH₂-Ph) and 22-28(PCy-C) and 13.6 (PCy₂-C) ppm; ¹⁹F {¹H} (356 MHz, THF-d₈) δ: −153.3 ppm.Anal. Calcd. for [FeC₃₇H₄₅P₂NO₂Br][BF₄]: C, 54.2; H, 5.5; N, 1.7. Found:C, 47.35; H, 5.39; N, 1.83**; MS (ESI, m/z⁺): 734.1 [FeC₃₇H₄₅P₂NO₂Br]+and 676.1 [FeC₃₅H₄₅P₂NBr]⁺ (loss of two —CO). IR: v(CO)=2004.4 cm⁻¹.

**Please note: It was observed that analysis of some complexes resultedin unsatisfactory carbon content, but acceptable hydrogen and nitrogencontent. The unsatisfactory carbon content was expected to be a resultof a combustion problem due to the tetrafluoroborate,hexafluorophosphate and tetraphenylborate anions, as has been previouslyreported in literature [Marcó, A.; Compañó, R.; Rubio, R.; Casals, I.Microchim. Acta 2003, 142, 13].

Discussion

Synthesis of iron(II)-P—CH═N—P′ complexes [Fe(Ph₂PCH₂CH₂N═CHCH₂PR₂)(NCCH₃)₃]²⁺, where R=Ph or Cy, has been previously reported, andinvolved a multi-component template synthesis using a cyclic phosphoniumsalt as a source of phosphine-aldehyde, [Fe(H₂O)₆][BF₄]₂, KOtBu and2-(diphenylphosphino)ethylamine in acetonitrile [Lagaditis, P. O.;Mikhailine, A. A.; Lough, A. J.; Morris, R. H. Inorg. Chem. 2010, 49,1094-1102]. These P—CH═N—P′-pincer ligands possessed inequivalentphosphorus donors, a feature which is not readily achieved byconventional means of synthesis of P—N—P-pincer ligands [Liang, L.-C.;Li, C.-W.; Lee, P.-Y.; Chang, C.-H.; Man Lee, H. Dalton Trans. 2011, 40,9004-9011; Liang, L.-C.; Chien, P.-S.; Lee, P.-Y. Organometallics 2008,27, 3082-3093; Lansing Jr., R. B.; Goldberg, K. I.; Kemp, R. A. DaltonTrans. 2011, 40, 8950-8958]. ³¹P{¹H} NMR spectra of both complexesdisplayed two AB doublets with a large ²J_(PP) coupling of 160 Hz forR=Ph and 148 Hz for R=Cy, which indicated that the P were trans; thusindicating a mer-arrangement of the P—CH═N—P′ ligand about iron.

This template synthesis was modified for the herein describedtechnology, and used FeBr₂ as an iron source (see FIG. 2). The synthesiswas conducted in THF under a N₂ atm to potentially facilitate isolatingan Fe(P—CH═N—P′)(Br)₂ complex; however, no reaction occurred untilexposure to carbon monoxide. Upon exposure, the reaction mixture (a paleyellow slurry) immediately turned red-purple. After stirring for asufficient time (5 h) and removing salts (KBr and excess FeBr₂) ared-purple solid was isolated. Starting with phosphonium dimer 1a whereR=Cy, iron compounds were produced in approximately a 1:1 ratio based on³¹P{¹H} NMR spectra. One set of doublets was observed at 67.8 and 71.9ppm (²J_(PP)=172 Hz) and a second set at 39.6 and 60.5 ppm (²J_(PP)=201Hz). Large coupling constants were indicative that the P atoms of theP—CH═N—P′ ligand were trans about the iron metal center. Similar resultswere obtained for 1b where R=iPr, and 1c where R=Ph (see paragraph[0063] above). Hence, two isomers of iron complexes were formed,trans-Br (2a-c) and cis-Br (3a-c) (see FIG. 2). Formation of twomer-Fe(P—CH═N—P′)(CO)(Br)₂ isomers contrasted with Milstein's systemwhere only a trans Br isomer was isolated [Langer, R.; Leitus, G.;Ben-David, Y.; Milstein, D. Angew. Chem. Int. Ed. 2011, 50, 2120-2124].

These results resembled those of Kirchner and co-workers who developed asynthesis of Fe(P—N—P)(CO)(X)₂ complexes, where X was Cl or Br, based onpincer ligand 2,6-(PiPr₂NH)₂C₅H₃N [Benito-Garagorri, D.; Alves, L. G.;Puchberger, M.; Mereiter, K.; Veiros, L. F.; Calhorda, M. J.; Carvalho,M. D.; Ferreira, L. P.; Godinho, M.; Kirchner, K. Organometallics 2009,28, 6902-6914; Benito-Garagorri, D.; Puchberger, M.; Mereiter, K.;Kirchner, K. Angew. Chem. Int. Ed. 2008, 47, 9142-9145]. They found thatwhen X was Cl, a cis isomer formed under solvent-free conditions, whilea trans isomer formed in solution. However, where X was Br, a mixture ofcis and trans isomers was always obtained. Consequently, an attempt tosynthesize a Cl analogue of 2 and 3 was made using a Cl salt of 1a andFeCl₂ in the template synthesis; however, both cis and trans isomersformed, as evidenced by ³¹P{¹H} NMR spectrum of the reaction mixture(see paragraph [0063] above). Without wishing to be bound by theory, itwas postulated that there was no control of cis and trans Br or Clisomers since a multi-component reaction was being employed where theligand was made in situ.

Consequently, compound Fe(CO)₄(Br)₂ [Benito-Garagorri, D.; Wiedermann,J.; Pollak, M.; Mereiter, K.; Kirchner, K. Organometallics 2007, 26,217-222] was tested as a starting iron source, with reasoning that ifhalides were already coordinated to iron, and CO ligands were removedunder UV light, one isomer could be selected for. When Fe(CO)₄(Br)₂ wasused in place of FeBr₂, there was an immediate release of gas as well asa color change to orange. A ³¹P{¹H} NMR spectrum of this mixturerevealed an intractable mixture. This was then exposed to UV light.After at least 5 h, the solution turned dark purple and a ³¹P{¹H} NMRspectrum of the solid isolated upon work up showed that the isomer at67.8 and 71.9 ppm (²J_(PP)=175 Hz) (R=Cy, R′=H) was the major specieswith on average less than 10% of other species. Crystals suitable forX-ray crystallography were isolated and confirmed selective formation ofthe trans-Fe(Cy₂PCH₂CH═NCH₂CH₂PPh₂)(CO)(Br)₂ complex, 2a (see FIG. 3).Complex trans-Fe(iPr₂PCH₂CH═NCH₂CH₂PPh₂)(CO)(Br)₂, 2b using phosphoniumdimer 1b was also made in this manner. However in the case ofphosphonium dimer 1c, a mixture of cis and trans isomers, 2c and 3c,were formed in every attempt.

These photochemical syntheses have several limitations, however, such aslow yields and formation of cis-[Fe(P—CH═N—P′)(CO)₂(Br)]⁺. The UVtemplate reaction was limited to only benzene or toluene as a solvent,as other solvents, such as THF or CH₂Cl₂ caused an increased amount ofother iron-containing complexes. Furthermore, the reactions only workedwell at small scales of less than 100 mg; otherwise, the amount of otherisomers increased. One attempted scale up lead to a selective synthesisof cis-[Fe(P—N—P′)(CO)₂(Br)]₂[FeBr₄] (cis-4a), as determined from X-raycrystallography, but this result was not reproducible (see FIG. 4D).This complex was isolated as the BPh₄ ⁻ salt in order to removeparamagnetic FeBr₄ ²⁻ anion and enable full characterization by NMRspectroscopy. Although a selective synthesis of cis-4a has beenunsuccessful thus far, characterization of this compound allowedidentification of its presence in crude products and distinguished the²J_(PP) constant from that of the trans-CO iron complexes 4a-d (FIG. 2).Reaction yields were low (on average 30%), but repeating the reaction atsmall scales repeatedly enabled a sufficient amount of the complexestrans-Fe(PCy-CH═N—PPh)(Br)₂(CO) and trans-Fe(P^(i)Pr—CH═N—PPh)(Br)₂(CO)to be obtained for the synthesis' next step. Unfortunately, in everyattempt to synthesize a hydride compound using either NaHBEt₃ or NaBH₄,intractable solids were isolated based on NMR spectra. As for LiAlH₄(with subsequent addition of alcohol), at least two iron-hydridecomplexes were observed in NMR spectra, but it was realized that theP—N—P′ ligand dissociated from the metal, at least to a certain extent,as negative chemical shift signals were observed. Hence, for thereaction of trans-Fe(PCy-CH═N—PPh)(Br)₂(CO) with LiAlH₄, followed bymethanol, ³¹P{1H} NMR spectrum showed signals at −2.28 and −20.22 ppm.Neither chemical shift corresponded to 2-(diphenylphosphino)ethylamine,nor to the phosphino-aldehyde, whose chemical shifts were −6 and −12ppm, respectively. Both negative signals were from reduced P—N—P′ligand, (Ph₂PC₂H₄)NH(C₂H₄PCy₂). Use of a tertiary alcohol, such ast-amyl alcohol, helped reduce ligand dissociation, but did notcompletely prevent it. Thus, complexes trans-[Fe(P—N—P′)(CO)₂(Br)][BF₄],4a-d, were focussed on, and the photochemical synthesis was not pursuedfurther.

Kirchner and co-workers reported formation oftrans-[Fe(P—N—P)(CO)₂(Br)][BF₄] compounds selectively from reaction of ahalide abstractor (such as AgBF₄) with a mixture of cis- andtrans-Fe(P—N—P)(CO)(Br)₂ isomers [Benito-Garagorri, D.; Alves, L. G.;Veiros, L. F.; Standfest-Hauser, C. M.; Tanaka, S.; Mereiter, K.;Kirchner, K. Organometallics 2010, 29, 4932-4942]. Consequently, thetemplate synthesis was adjusted to include AgBF₄, following theinclusion of which there was an immediate color change from red-purpleto bright purple, the color of complexes 4a-c (FIG. 2). ³¹P{¹H} NMRspectra of isolated compounds showed formation of one new iron complexwith AB doublets with smaller ²J_(PP) couplings of 82 Hz (R=Cy), 85 Hz(R=iPr) and 94.5 Hz (R=Ph), significantly smaller than that ofcis-[Fe(P—CH═N—P′)(CO)₂(Br)]⁺ with ²J_(PP)=145 Hz (see [00104]).Crystals of 4a and 4b suitable for X-ray crystallography were obtained(see above) and confirmed that trans-CO geometry about iron had alsooccurred. Without wishing to be bound by theory, it was considered thatthe mechanism for this selective trans configuration may have followedone proposed by Kirchner and co-workers, given that the reaction couldalso occur in the absence of carbon monoxide [Benito-Garagorri, D.;Alves, L. G.; Veiros, L. F.; Standfest-Hauser, C. M.; Tanaka, S.;Mereiter, K.; Kirchner, K. Organometallics 2010, 29, 4932-4942]. ¹³C{¹H}NMR spectra of complexes 4a-c displayed only one broad triplet or adoublet of doublets (dd) for CO ligands at around 210 ppm, whichindicated that the complexes have C_(s) symmetry, as expected. A ¹³COanalogue of 4b (henceforth referred to as 4b-¹³CO) was synthesized tofurther confirm, via NMR spectroscopy, that two CO ligands werecoordinated to iron. ³¹P{¹H} NMR (CD₂Cl₂) spectrum of 4b-¹³CO displayedthe same two phosphorus resonances as 4b at 45.47 and 78.31 ppm(²J_(PP)=85.9 Hz, ²J_(PC)=21.8 Hz), but with a triplet of doubletmultiplicity instead of a doublet for each phosphorus atom. The ¹³C{¹H}NMR spectrum of 4b-¹³CO displayed one intense CO triplet resonance at211.72 ppm (²J_(CP)=24.0 Hz).

Chiral amino-phosphine ligand,(S,S)-2-amino-1-phenylpropyldiphenylphosphine, was successfully used inthe template synthesis method to generate a chiral complex: (S,S)-4d.(S,S)-4d was designed with Cy substituents on the ligand in anticipationthat bulky groups would enhance enantiomeric interactions withsubstrates during catalysis. ¹³C{¹H} NMR spectrum of (S,S)-4d showed twoinequivalent CO resonances with a dd pattern at 210.5 and 214.7 ppm. IRspectra of complexes 4a-d displayed only one ν_(CO) absorption in arange of 2000 to 2011 cm⁻¹, similar to those of trans-CO iron complexesreported by Kirchner.

Example 2 Catalytic Hydrogenation Experimental

All hydrogenation reactions were performed at constant pressures using astainless steel 50 mL Parr hydrogenation reactor. Temperature wasmaintained at 50° C. using a constant temperature water bath. Reactorwas flushed several times with hydrogen gas at 5 atm prior to additionof catalyst and substrate, and base solutions. For standard catalysiswith in situ prepared catalysts, a vial was charged with[Fe(CO)₂(Br)(P—CH═N—P′)]BF₄ (5 mg, 0.006 mmol) and 3 mL THF. To thissolution, 0.05 mL of LiAlH₄ (1M in THF) was added and the solution'scolor immediately changed from purple to a golden brown. After stirringfor 5 min, 2-methyl-2-butanol (0.5 mL) was added; the solution wasallowed to stir for 10 min or until gas evolution ceased. The solutionwas transferred to a syringe equipped with a 12 inch needle. The samevial was then charged with substrate (6.095 mmol) and 3 mL THF. Thesolution was taken up into the same syringe that already contained theprecatalyst solution; the needle was then stoppered. For catalysis withpre-formed species 7b and (S,S)-8d (see [00133-00134]), NMR solutions(THF-d₈) were transferred to a vial in a nitrogen filled glovebox andTHF (6 mL) and substrate (6.095 mmol) were added. The solution wastransferred to a syringe equipped with a 12 inch needle and stoppered. Asecond vial was charged with KOtBu (10 mg, 0.089 mmol) and 3 mL THF.This solution was transferred to a second syringe equipped with a 12inch needle and stoppered as well. Both syringes were taken out of theglove box and injected into the prepared Parr reactor against a flow ofhydrogen gas. Small aliquots of the reaction mixture were quicklywithdrawn with a syringe and needle under a flow of hydrogen at timedintervals. Alternatively, small aliquots of the reaction mixture weresampled from a stainless steel sampling dip tube attached to a modifiedParr reactor. The dip tube was 30 cm in length with an inner diameter of0.01 inches, and a swing valve was attached to the end of the samplingtube. All samples for gas chromatography (GC) analyses were diluted to atotal volume of approximately 1 mL using oxygenated ethanol. Allconversions were reported as an average of two runs. Reportedconversions were reproducible. Conversion and enantiomeric excess ofhydrogenated ketones were analyzed by a Perkin-Elmer Clarus 400chromatograph, equipped with a chiral column (CP chirasil-Dex CB 25m×2.5 mm) and auto-sampling capability. Hydrogen was used as a mobilephase at a column pressure of 5 psi with a split flow rate of 50 mL/min.Injector temperature was 250° C. and FID temperature was 275° C. Oventemperatures and retention times (t_(SM), t_(R), t_(S)/min) forsubstrates were as follows^(a):

Oven Temp. t_(SM) t_(R) t_(S) Substrates/Alcohol Products (° C.) (min)(min) (min)

130  4.83  8.09  8.43

145  4.96 11.07 12.96

140  6.51 14.78 15.78

145  6.52 12.10 13.26

155  7.02 12.79 13.26

140 11.44 15.0 15.64

130  7.12 10.63 11.4

120  8.9 19.19 19.8

140 (90)  5.28 (36.18) 10.29 (181.83) 10.29 (192.99)

170  9.67 14.43 14.76

180  7.94 12.50

130 (90)  6.81 (81.68)  7.80 (164.50)  7.80 (153.85)

150 23.1 38.3 40.1

140 24.1 74.2 86.2

125 11.49 18.68 19.49

130  8.29 12.22 11.61

 60  3.80  9.80 10.37

115  8.25 16.26 17.57

 90  6.90 15.41 15.88

n/a^(a)  9.64 12.11 12.97

 90  3.43  5.63  5.63

130  3.53 7.31

n/a^(a) 10.25 13.22 17.02 ^(a)HPLC conditions: isopropanol:hexanes =10:90, flow rate = 0.5 mL/min.

Discussion

Discovery of precatalysts 4a-d activity towards hydrogenation was madewhile treating the iron dicarbonyl complexes with various hydridereagents to produce iron hydride amine complexes with a bifunctionalHN—FeH group known to reduce C═O and C═N bonds [Morris, R. H. Chem. Soc.Rev. 2009, 38, 2282-2291; Mikhailine, A. A.; Maishan, M. I.; Lough, A.J.; Morris, R. H. J. Am. Chem. Soc. 2012, 134, 12266-12280; Zuo, W.; Li,Y.; Lough, A. J.; Morris, R. H. Science 2013, 342, 1080-1083]. Reactionswith NaHBEt₃ or NaBH₄ formed intractable mixtures; LiAlH₄ was morepromising. To generate iron hydrides, often a precursor was reacted witha slight excess of LiAlH₄ in THF [Antberg, M.; Frosin, K. M.;Dahlenburg, L. J. Organomet. Chem. 1988, 338, 319-327; Antberg, M.;Dahlenburg, L. Z. Naturforsch., B: Chem. Sci. 1985, 40, 1485-1489;Roger, C.; Marseille, P.; Salus, C.; Hamon, J.-R.; Lapinte, C. J.Organomet. Chem. 1987, 336, C13-C16; Field, L. D.; Messerle, B. A.;Smernik, R. J.; Hambley, T. W.; Turner, P. Inorg. Chem. 1997, 36,2884-2892; Liu, T.; Chen, S.; O'Hagan, M. J.; Rakowski DuBois, M.;Bullock, R. M.; DuBois, D. L. J. Am. Chem. Soc. 2012, 134, 6257-6272];sometimes with the addition of a protic solvent [Gao, Y.; Holah, D. G.;Hughes, A. N.; Spivak, G. J.; Havighurst, M. D.; Magnuson, V. R.;Polyakov, V. Polyhedron 1997, 16, 2797-2807; Ohki, Y.; Suzuki, H. Angew.Chem. Int. Ed. 2000, 39, 3120-3122; Argouarch, G.; Hamon, P.; Toupet,L.; Hamon, J.-R.; Lapinte, C. Organometallics 2002, 21, 1341-1348;Sellmann, D.; Weber, W. J. Organomet. Chem. 1986, 304, 195-201; Jia, G.;Drouin, S. D.; Jessop, P. G.; Lough, A. J.; Morris, R. H.Organometallics 1993, 12, 906-916]. For at least some the hereindescribed complexes, it was found that addition of at least 6 equiv. ofLiAlH₄ was necessary to produce a solution of iron hydride complexesreproducibly (see paragraph [00130]). The mixture was then treated withalcohol until gas evolution had ceased. Methanol, ethanol and tert-amylalcohol (2-methyl-2-butanol, tAmylOH) were found to give active catalystpreparations. Then ketone or imine substrates and additional THF wereadded and the entire solution was injected into a prepared pressurereactor. This procedure allowed facile and efficient screening ofoptimal conditions for catalysis (see Table 1 below).

While 25 atm H₂ was an effective pressure for catalysis, 5 atm H₂ stillresulted in full conversion of acetophenone to 1-phenylethanol within 10min. Faster conversions were observed at 50° C. than at 25° C. (Table 1,Entry 2 vs 3). Substitution of LiAlH₄ with NaAlH₄ did not affectreaction rate (Entry 4). No catalysis was observed in absence of base(Entry 7), which prevented testing of base sensitive substrates. Use ofa tertiary alcohol, tAmylOH, in the catalyst activation process createda more active system than with primary alcohols, MeOH or ethanol (Entry2 vs 5 vs 6). Without wishing to be bound by theory, two possibleexplanations for the alcohol effect were considered: the catalystactivation period was slower with methoxide or ethoxide as a ligand; or,there were other deactivation processes occurring that were morepronounced with a primary alkoxide [Fiedler, A.; Schroder, D.; Schwarz,H.; Tjelta, B. L.; Armentrout, P. B. J. Am. Chem. Soc. 1996, 118,5047-5055]. Use of a tertiary alcohol also had prevented non-selectivetransfer hydrogenation mediated by Al(OR)₃. It appeared that the P—N—P′ligand to containing at least one large alkyl-substituted phosphorusatom was beneficial for creating an active catalyst (Table 1, Entry 1and 2 vs 9). TOF value for acetophenone hydrogenation using the 4aprecatalyst (Entry 8, TON 2000, TOF 1980 h⁻¹ at 50° C. and 5 atm H₂) wascomparable to that reported for Milstein's complexes M2, 430 h⁻¹ at 40°C., 4 atm H₂. One difference observed with the herein describedcomplexes was that THF could be used in place of alcohol solvents neededfor catalysis with M2 or M3 (FIG. 1).

For benzaldehyde hydrogenation, it was found that pressures of 10 atm H₂gas facilitated achieving 90% conversion in 2.5 h versus 10 h with 5 atmH₂ gas:

Substrate scope of asymmetric ketone hydrogenation reaction wasinvestigated at 50° C. and 5 atm H₂ using 0.1 mol % of the activatedmixture prepared from (S,S)-4d (see Table 2 below). Many aryl ketoneswere converted to alcohols with good enantioselectivity, typically about80% (S). This system appeared to offer a higher activity towardsenantioselective ketone hydrogenation over other iron-based catalysts(FIG. 1) given that it employed H₂ gas. It was interesting to note thatthe (S,S) chiral catalyst system produced (S) alcohol enantiomers,wherein (R) enantiomers of alcohols are generally expected when usingligands derived from (S,S)-diamine [Noyori, R.; Ohkuma, T. Angew. Chem.Int. Ed. 2001, 40, 40-73; Abdur-Rashid, K.; Faatz, M.; Lough, A. J.;Morris, R. H. J. Am. Chem. Soc. 2001, 123, 7473-7474; Clapham, S. E.;Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201-2237].

A catalyst mixture starting with (S,S)-4d was tested for acetophenonehydrogenation at both 50° C. and 25° C. Enantioselectivity of thereaction increased from 80% to 89% (S), while time for completeconversion lengthened from 30 min to 90 min, respectively. The catalyticmixture at 50° C. also accommodated additional loadings of substrate:four additional batches of acetophenone (1000 equiv.) were added at 30minute intervals without slowing down the catalyst or affecting the eeof product (S)-1-phenylethanol. A fifth batch slowed the catalyticreaction but it still went to completion. Enantioselectivity of the(S,S)-4d system decreased along with activity as steric hindrance due tobulky substituents on the ketone substrate increased (see Table 2,Entries 7 vs 8 vs 9). Low enantioselectivity was not due to racemizationas the reactions were monitored periodically, and ee remained constantuntil completion. For less bulky substrates, such as benzylacetone(Entry 15), catalytic reduction by the (S,S)-4d system was rapid butenantioselectively poor (ee=5%). For substrate 1-phenyl-2-butanone(Entry 16), which was more sterically hindered than benzylacetone, eeincreased to 30%, while there was a substantial decrease in catalyticactivity (TOF=90 h⁻¹). This lower catalytic activity was considered tobe a result of enolate formation due to presence of base. Fortunately,the inhibitory effect of enolates did not poison the catalystcompletely. Pre-catalyst (S,S)-4d was found to give moderately efficienthydrogenations of non-aromatic ketones (Entry 17) but with inferiorenantioselectivity (ee=46%). The system was also found to catalyzereduction of 2-acetylthiophene (Entry 18) and 2-acetylfuran (Entry 19)to near completion. However, catalysis was slower (TOF=240 h⁻¹ for2-acetylthiophene and 220 h⁻¹ for 2-acetylfuran) when compared toacetophenone (TOF=990 h⁻¹). The difference was considered to be due tothe heterocycle's heteroatom binding to the iron; however; sincecatalysis did go to completion, it was postulated that the chelationeffect may be reversible. The chelation effect appeared to be morepronounced for substrate 2-acetylpyridine (Entry 20), such thatcatalysis ceased at 20% conversion (or 200 TON) after one hour. It wasconsidered that the substrate, its alcohol product, or both, deactivatethe catalyst. An increase in H₂ pressure to 10 atm enabled thehydrogenation of 2-acetylpyridine to 60% conversion (or 600 TON) in twohours and maintained a 74% ee of the hydrogenated product.

The (S,S)-4d system did not hydrogenate trans-4-phenyl-3-buten-2-one,unsaturated ketone, at 5 or 10 atm H₂ pressure (Table 2, Entry 21). Anolefin group did not poison the catalyst, because the (S,S)-4d systemwas able to fully hydrogenate 5-hexen-2-one (Entry 22) to 5-hexen-2-olwithout affecting the olefin as determined by NMR spectroscopy. However,in comparison with benzylacetone (Entry 15), where catalytichydrogenation was complete in one hour, hydrogenation of 5-hexen-2-onewas complete in four hours. Furthermore, upon addition of 1-hexene (500equiv) to a hydrogenation reaction of acetophenone (1000 equiv),catalysis was complete in 90 min as opposed to 30 min. These resultsimply there may be some degree of reversible olefin coordinationanalogous to the situation with heterocyclic substrates (Entries 18-20).Enone and diene compounds of iron are known and have been well studied[Brookhart, M.; Nelson, G. O. J. Organomet. Chem. 1979, 164, 193-202;Moulton, B. E.; Duhme-Klair, A. K.; Fairlamb, I. J. S.; Lynam, J. M.;Whitwood, A. C. Organometallics 2007, 26, 6354-6365; Knolker, H.-J.Chem. Rev. 2000, 100, 2941-2962; Russell, S. K.; Milsmann, C.;Lobkovsky, E.; Weyhermuller, T.; Chirik, P. J. Inorg. Chem. 2011, 50,3159-3169]. It was considered that there was some degree of reversiblecoordination with enones and dienes, as well with the presently claimediron catalysts because, upon addition of trans-4-phenyl-3-buten-2-one(500 equiv) to a hydrogenation reaction of acetophenone (1000 equiv),catalysis was complete in 90 min with zero conversion of the enoneadditive.

Additional chiral complexes were tested for acetophenone hydrogenationat 50° C., and enantioselectivity for each reaction was evaluated.Results are outlined below (Table 2a).

Hydrogenation of imines was also investigated using activated (S,S)-4dunder harsher conditions of 20 atm H₂ gas and at 50° C. at substrateloadings of 1 mol % (based on (S,S)-4d) and 10 mol % base. The systemwas found to be inactive for hydrogenation of imine substrates,N-(1-phenylethylidene)aniline, orphenyl-N-(1-phenylethylidene)methanamine, or nitrile substratebenzylnitrile. However, hydrogenation of activated imine,N-(diphenylphosphonyl)propiophenoneimine was observed in 22 h with a TOFof 5 h⁻¹:

Activity of this system was observed to be higher than that of Bellerand co-worker's cooperative system B2, which had a TOF of about 1 h⁻¹under harsher conditions [Zhou, S.; Fleischer, S.; Junge, K.; Beller, M.Angew. Chem. Int. Ed. 2011, 50, 5120-5124]. Enantioselectivity ofactivated (S,S)-4d was high (ee=90% (S)).

Example 3 Study of Catalyst Activation Steps Experimental

General Procedure for Synthesis of Complexes 5a-d and 6a-d: A vial wascharged with [Fe(CO)₂(Br)(P—CH═N—P′)][BF₄] (˜20 mg) in THF (5 mL) toyield a bright purple solution to which LiAlH₄ was added until thesolution turned dark yellow-brown (˜20 mg). After stirring for 10 min,solvent was removed and a residue was taken up with ether (5-10 mL) toremove a grey-black precipitate. 12-crown-4 (˜4-5 drops) was added tocause precipitation of an off-white solid ([Li(12-crown-4)]BH₄]).Resulting solution was filtered and dried in vacuo to yield a yellowresidue.

Chemical Shifts and Coupling Constants of Compounds from Reaction of4b-¹³CO with LiAlH₄:

Nucleus 5b-¹³CO 6b-¹³CO Unidentified minor product (THF-d₈) (ppm, ²J Hz)(ppm, ²J Hz) (ppm, ²J Hz) ¹H NMR −10.9 (br. s) −11.2 (m) −11.7 (600 MHz)−13.2 (tdd) −11.6 (m) (detected via ¹H-³¹P HMBC) J_(HH) = 8 J_(HH) = 24Hz J_(HP) = 54 J_(HC) = 12 ³¹P{¹H} NMR 86.8 (dd) 83.6 (td) 57.2 (d) (243MHz) 107.5 (dd) 100.7 (td) 75.7 (d) J_(PP) = 105 J_(PP) = 20 J_(PP) =138 J_(PC) = 27 J_(PC) = 28 ¹³C{¹H} NMR 221.4 (t) 220.3 (t) 220.1 (br.s) (150 MHz) J_(CP) = 26 J_(CP) = 39.0 220.4 (br. s)

NMR Spectroscopy Chemical Shifts of Iron Hydride-AluminumhydrideCompounds with Coupling Constants (600 MHz, THF-d₈):

5 6 (trans-dihydride) (cis-dihydride) ¹H NMR ³¹P NMR ¹H NMR ³¹P NMR(ppm, ²J Hz) (ppm, ²J Hz) (ppm, ²J Hz) (ppm, ²J Hz) 5a/6a −10.9 (br. s)86.3 (d) −11.2 (m) 83.0 (d) R = Cy, −13.2 (td) 99.1 (d) J_(HH) = 23 91.1(d) R′,R″ = H J_(HH) = 8 J_(PP) = 105 J_(HP) ≈ 27, 37 ^(a) J_(PP) = 20J_(HP) = 54 −11.7 (m) J_(HH) = 23 J_(HP) ≈ 28, 34 ^(a) 5b/6b −10.9 (br.s) 86.8 (d) −11.2 (m) 83.6 (d) R = iPr, −13.2 (td) 107.5 (d) J_(HH) = 24100.7 (d) R′,R″ = H J_(HH) = 8 J_(PP) = 105 J_(HP) ≈ 28, 38 ^(b) J_(PP)= 20 J_(HP) = 54 −11.6 (m) J_(HH) = 24 J_(HP) ≈ 31, 37 ^(b) 5c/6c −10.6(br. s) 90.0 (s) −10.9 (m)^(c) 86.4 (s) R = Ph, −12.6 (td) R′,R″ = HJ_(HH) = 8 J_(HP) = 56 (S,S)- −10.9 (br. s) 97.8 (d) −10.5 (m) 83.9 (d)5d/6d −12.5 (td) 98.3 (d) J_(HH) = 23 102.1 (d) R = Cy, J_(HH) = 8J_(PP) = 102 J_(HP) ≈ 29, 34 ^(d) J_(PP) = 21 R′ = Me J_(HP) = 54 Hz−11.6 (m) R″ = Ph J_(HH) = 23 −10.8 (m) 89.65 (d) J_(HH) = 22 92.39 (d)−11.7 (m) J_(PP) = 21 J_(HH) = 22 ^(a) See FIG. 5. ^(b) See FIG. 6.^(c1)H NMR (C6D6): −9.5 (m), −10.6 (m) ppm. ^(d) See FIG. 7.

General Procedure for the Synthesis of Complexes 7 and 8:

Following the procedure as outlined for 5a-d and 6a-d, an excess ofalcohol (MeOH or tAmylOH) was added dropwise to a final Et₂O solutionuntil gas evolution ceased (˜10 drops). The solution, originally yellow,turned orange. The solvent was removed and a residue was taken up withpentane and filtered. The solution was dried in vacuo to afford anorange residue.

NMR Chemical Shifts (THF-d⁸) of Iron Pre-Catalysts of 7 and 8:

¹H NMR (600 MHz) (ppm, ²J Hz) ³¹P{¹H} NMR (243 MHz) (ppm, ²J Hz) ¹³C{¹H}NMR (150 MHz) (ppm, ²J Hz) 7b-¹³CO −18.6 (td) 82.8 (ddd) 222.3 (td)isomer I J_(HC) = 19 102.1 (ddd) J_(CH) = 14 (R′′′ = Me) J_(HP) = 52J_(PH) = 39^(a) J_(CP) = 26 J_(PC) = 27 J_(PP) = 119.1 7b-¹³CO −21.6(td) 77.2 (ddd) 224.1 (td) isomer II J_(HC) = 20 96.7 (ddd) J_(CH) = 17(R′′′ = Me) J_(HP) = 57 J_(PH) = 41^(a) J_(CP) = 27 J_(PC) = 27 J_(PP) =137 7b-CO −22.7 (dd) 75.0 (dd) n/a isomer III J_(HP) = 52, 56 95.7 (dd)(R′′′ = Me) J_(PH) = 21^(a) J_(PP) = 136 8b −21.6 (dd) 75.3 (dd) n/aisomer I J_(HP) = 52, 56 94.8 (dd) (R′′′ = tAmyl) J_(PH) = 30^(a) J_(PP)= 137 8b −26.6 (dd) 64.7 (dd) n/a isomer II J_(HP) = 60, 72 86.6 (dd)(R′′′ = tAmyl) J_(PH) = 53 J_(PP) = 152 (S,S)-8d −21.6 (dd) 95.4 (dd)n/a isomer I J_(HP) = 47, 56 86.4 (dd) (R′′′ = tAmyl) J_(PH) = 49^(a)J_(PP) = 140 (S,S)-8d −24.3 (t) 108.5 (d) n/a isomer II J_(HP) = 48 98.1(d) (R′′′ = tAmyl) J_(PP) = 148 ^(a)Residual coupling due to incompletedecoupling of the high field hydride resonance.

NMR Spectroscopy Chemical Shifts of Minor Iron Mono-Hydride Compounds of(S,S)-8d as Determined from ¹H—³¹P HMBC NMR Spectrum:

Minor isomer i Minor isomer j Minor isomer k ¹H NMR −24.2 (t) −24.6 (dd)−25.2 (t) (ppm, ²J Hz) (J_(HP) = 54) (J_(HP) = 54, 64) (J_(HP) = 48) ³¹PNMR 86.0 83.5 101.5 (ppm) 88.8 91.3 107.0

Density Functional Theory Calculations on Models of 5 and 6: DFTcalculations were performed using Gaussian 09 [Frisch, M. J. et al.,2009, Gaussian 2009 Revision B. 2001]. M06 hybrid functional was usedfor all calculations [Zhao, Y.; Truhlar, D. Theor. Chem. Acc 2008, 120,215-241; Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157-167].All atoms were treated with a 6-31++G(d,p) basis set. A pruned (99,590)integration grid was used throughout (Grid=UltraFine). Optimizationswere performed in tetrahydrofuran using an integral equation formalismpolarizable continuum model (IEF-PCM) with radii and nonelectrostaticterms from a SMD solvation model [Marenich, A. V.; Cramer, C. J.;Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378-6396]. Full vibrationaland thermochemical analyses (1 atm, 298 K) were performed on optimizedstructures to obtain solvent-corrected free energies (G°) and enthalpies(H°). Optimized ground states were found to have zero imaginaryfrequencies.

DFT Calculations (GAUSSIAN09/M06/6-31++G(d,p)/IEF-PCM (THF))—Coordinatesfor Complex FeH₂(CO)(PH₂CH₂CH₂N(AlH₂)CH₂CH₂PH₂) (Model of Compounds 5b;See FIG. 8):

Fe −0.03315 0.69757 −0.02087 P 2.16119 0.61500 −0.25240 P −2.208370.41626 −0.26697 N 0.06413 −1.38257 −0.10934 C 2.54387 −1.19015 −0.36182H 3.36300 −1.39081 −1.05957 H 2.86631 −1.51414 0.63422 C 1.27051−1.89172 −0.79469 C −2.42763 −1.41778 −0.31602 H −0.04300 0.608301.63923 H −0.02578 0.75183 −1.55863 C −1.10464 −2.01380 −0.75820 H−3.23968 −1.71509 −0.98710 H −2.69748 −1.73768 0.69685 H −1.00009−1.87992 −1.84847 H −1.09850 −3.09988 −0.56733 H 1.36461 −2.97796−0.63029 H 1.13193 −1.74285 −1.87913 Al 0.07311 −1.11553 1.78927 H1.46623 −1.45158 2.49243 H −1.24674 −1.62946 2.52569 C −0.11963 2.441080.06728 O −0.18343 3.60160 0.10271 H −2.82974 0.88514 −1.44282 H−3.17979 0.83008 0.67248 H 2.74731 1.18103 −1.40316 H 3.08420 1.078420.71276 Electronic Energy = −2518.60453821 Sum of electronic and thermalEnergies = −2518.391616 Sum of electronic and thermal Enthalpies =−2518.390672 Sum of electronic and thermal Free Energies = −2518.446116

DFT Calculations: Coordinates forLi[FeH₂(CO)(PH₂CH₂CH₂N(AlH₃)CH₂CH₂PH₂)] (Model of Compounds 6b; See FIG.9A):

Fe −0.64232 0.39347 −0.00739 H −0.31074 1.46348 1.09604 H −0.296021.43344 −1.13612 P −0.72942 −1.13867 −1.61558 H −1.05748 −0.78729−2.94396 H −1.47121 −2.34205 −1.53194 P −0.74632 −1.07025 1.65952 H−1.01712 −0.64929 2.98038 H −1.53015 −2.24914 1.65160 C −2.30716 0.93939−0.02911 O −3.40041 1.33393 −0.04605 C 0.98646 −1.70829 1.73762 H1.26347 −2.03048 2.74798 H 1.04015 −2.58485 1.07914 C 1.02262 −1.72290−1.72520 H 1.28812 −2.03002 −2.74324 H 1.12388 −2.60070 −1.07442 C1.87153 −0.57047 1.23439 H 2.90162 −0.93076 1.08672 H 1.92082 0.199582.02028 C 1.87965 −0.55959 −1.23394 H 2.92118 −0.88918 −1.09412 H1.90002 0.20973 −2.02112 N 1.38475 0.06076 0.00122 H 2.20292 2.47366−1.24969 H 2.18376 2.48351 1.24953 Al 1.15958 2.02865 −0.00631 H 0.527773.64552 −0.01930 Li 2.32079 4.11671 −0.00849 Electronic Energy =−2526.538617 Sum of electronic and thermal Energies = −2526.522415 Sumof electronic and thermal Enthalpies = −2526.521471 Sum of electronicand thermal Free Energies = −2526.581234

General Procedure for the Synthesis of Complex 9: While it was foundthat base was required for catalysis (see [00118]), methoxide hydridecomplexes 7b were unstable in base when a substrate and hydrogen wasabsent. When KOtBu was added to a C₆H₆ solution of prepared pre-catalyst7b, the orange solution turned red after stirring for 10 min. Afterremoval of a white precipitate, a ³¹P{¹H} NMR (C₆D₆) spectrum of theisolated red residue showed one major compound as two doublets at 79.7and 101 ppm (J_(PP)=73.0 Hz), along with minor compounds and freeP—CH₂NH—P′ ligand. Once crystals suitable for X-ray diffraction wereisolated by slow diffusion of pentane into the C₆D₆ solution, the majorcompound was identified to be a neutral Fe(0) complex:Fe(Ph₂PCH₂CH₂NHCH₂CH₂P^(i)Pr₂)(CO)₂, 9:

FIG. 9B depicts the ORTEP plot of 9 (thermal ellipsoids at 30%probability), wherein hydrogen atoms of phenyl and iso-propylsubstituents were removed for clarity, and selected bond lengths (Å) andangles (°) included Fe(1)-C(6): 1.723(2); Fe(1)-C(5): 1.797(2);Fe(1)-N(1): 2.088(2); Fe(1)-P(2): 2.1735(6); Fe(1)-P(1): 2.2038(6);C(6)-Fe(1)-C(5): 93.94(9); C(6)-Fe(1)-N(1): 172.41(9); C(5)-Fe(1)-N(1):93.07(8); C(6)-Fe(1)-P(2): 89.62(7); C(5)-Fe(1)-P(2): 120.34(6);C(5)-Fe(1)-P(1): 119.37(6); P(2)-Fe(1)-P(1): 119.55(2); N(1)-Fe(1)-P(2):84.34(5); C(6)-Fe(1)- P(1): 95.00(7).

Discussion

Synthesis of hydride-aluminumhydride iron complexes 5a-d, 6a-d andmonohydride complexes 7b (R′″ Me) and 8b and (S,S)-8d (R′″ tAmyl) beganwith purple THF solutions of 4a-d, which turned immediately dark brownand evolved gas upon addition of a LiAlH₄ activator:

Removal of THF and subsequent removal of a black precipitate with Et₂Oafforded a dark brown-yellow solution. The isolated residue was examinedby NMR spectroscopy and found to contain two ironhydride-aluminumhydride species with either trans hydrogens with amer-P—N—P′ ligand on iron (5a-d), or cis hydrogens with a fac-P—N—P′ligand on iron (6a-d) as shown above. For example, 5b was thought tohave a mer configuration because it had a large J_(PP) of 105 Hz, while6b was thought to be fac with a small J_(PP) of 20 Hz. Since no protonsource was added to the reaction, it was postulated that Al was stillbound to the amide as AlH₃. ²⁷Al NMR spectroscopy supported thispostulate, since it displayed a large broad signal at 100 ppm; as did IRspectroscopy, which had vAl-H absorptions at 1783 and 1648 cm⁻¹ [Ares,J. R.; Aguey-Zinsou, K. F.; Porcu, M.; Sykes, J. M.; Dornheim, M.;Klassen, T.; Bormann, R. Mater. Res. Bull. 2008, 43, 1263-1275].Compound H₂Al[N(CH₂CH₂NMe₂)₂]AlH₃ provided a precedent for such analane-amide adduct [Luo, B.; Kucera, B. E.; Gladfelter, W. L. DaltonTrans. 2006, 0, 4491-4498]. It was considered that there were up to twoconfigurations of structures 5 (H trans to H, and H trans to N);however, only trans is the major structure, as drawn. For structures 6,there was only one configuration and its enantiomer for 6a and 6b, andonly one for 6c (no enantiomer), but there were two diastereomers for(S,S)-6d with the nitrogen in a R or S configuration. Chemical shifts,coupling constants, as well as DFT calculations (see above), were usedto support the tentative structures of iron hydride species 5 and 6, asshown above.

For the DFT analysis of structures 5 and 6, simplified structures wereemployed, wherein hydrogen replaced substituents on phosphorus, as shownin FIGS. 8 (for hydride-aluminumhydrides 5) and 9 (for 6). Transhydride-aluminumhydrides had Fe—H distances of 1.54 and 1.66 Å, thelatter due to an interaction with aluminum. Interaction of the hydridewith quadrupolar aluminum at Al—H 1.73 Å explained the broadening of thecorresponding hydride resonance at −10.9 ppm in the ¹H NMR spectrum of5b. Cis hydride-aluminumhydrides of complexes 6 had Fe—H distances of1.57 Å, and those hydrogens were at longer distances from aluminum (Al—H1.92, 1.94 Å), which explained why those resonances were not as broadand appeared as pseudo AA′XX′ patterns in the region −11 to −12 ppm dueto the arrangement of similar hydride environments and phosphorusenvironments in an approximately square plane around the octahedron ofiron. Li+ was calculated as ion-paired to AlH₃ unit, but in THF solutionit might have also had a THF coordinated; as noted, Li+ was detected byNMR.

One observation was that complexes 5c and 6c formed from startingcomplex 4c contained symmetrical PPh₂CH₂CH₂NCH₂CH₂PPh₂ ligands. This wasapparent from a ³¹P{1H} NMR spectrum where 5c and 6c gave singlets dueto equivalent phosphorus nuclei (see FIG. 11). This was consideredevidence that the imine group was being reduced in complexes 4a-d duringcatalyst activation.

It was observed that catalytic hydrogenation activity may depend on thenature of the alcohol used, and as such, treatment of the iron hydridealuminumhydride complexes with MeOH and tAmylOH were separately examinedby NMR. Upon addition of an excess of alcohol to a C₆H₆ or THF solutionof complexes 5 and 6, iron pre-catalysts mer Fe(H)(OR′″)(CO)(P—CH₂NH—P′)(7 with R′″=Me; 8 with tAmyl) were formed (see [00141]). The solution'scolor changed from yellow to orange along with gas evolution. NMRspectra of the isolated residues showed multiple iron hydride complexeswith similar patterns; however, they were iron mono-hydride complexes.¹H, ³¹P and ¹³C NMR spectra of complexes 7b, 7b-¹³CO, 8b and (S,S)-8dwere examined in detail to distinguish differences between reaction withMeOH as compared with that of ^(t)AmylOH (see paragraph [00134]).³¹P{¹H} NMR spectrum demonstrated that there was no dissociation of theP—N—P′ ligands and that there was downfield shift of the phosphorusresonances for solutions of 8 versus solutions of 7, thus providingevidence for an alkoxide ligand. Major iron monohydride compounds werepostulated to have a mer structure with trans phosphorus groups of theP—N—P′ ligand, since ²J_(PP) were in a range of 120-150 Hz. ²J_(HP)values of the ¹H NMR spectrum hydride resonances were greater than 50Hz, which was also consistent with structures with hydrides cis tophosphorus (see [00134]). ¹H—³¹P HMBC NMR spectrum enabled correlationof hydride resonances to appropriate phosphorus resonances for 7b,7b-¹³CO and (S,S)-8d. ¹H-¹³C HMBC NMR spectrum of 7b-¹³CO showed uniquecorrelations for each hydride to one CO ligand.

It was considered, therefore, that each isomer had a mer-configurationwith one P—NH—P′, hydride, alkoxide and carbonyl ligand. Trans positionsfor the monodentate ligands were inequivalent because of the NH group'sposition. Consequently, it was determined that the total number ofpossible methoxide isomers were: six for 7a and 7b (and sixenantiomers), three for 7c and six for (S,S)-7d. ^(t)Amyl derivatives of8 had the same numbers of isomers (see DFT analysis below).

Additionally, relative ratios of various hydride species 7b appeared tovary slightly depending on exact reaction conditions: use of slightlyless alcohol appeared to favour formation of hydride species with ¹H NMRsignals at −18.6 ppm (see [00134]); whereas use of excess alcoholyielded much less of the −18.6 ppm hydride species, and a new specieswith ¹H NMR hydride signals at −22.7 ppm. Existence of a separatespecies which did not correlate to any hydrides was also detected, andwas assigned as zero-valent Fe(CO)₂(P—CH₂NH—P′) complex 9 with ³¹P{¹H}doublets at 101.0 and 79.7 ppm with J_(PP) of 73 Hz. Formation of thiscomplex was associated with presence of base (see [00139]), in this caseLiOH, which was likely produced by hydrides reacting with traces ofwater in the solvents. The presence of this complex showed that ligandredistribution reactions were possible. Over a period of 24-48 hours,the hydride signals at −18.6 ppm, and all minor hydride species,disappeared, leaving a spectrum with only two hydride species at −21.6and −22.7 in a 1:1 ratio. ¹H—³¹P HMBC and ¹H-¹³C HMBC enabledcorrelation of the hydride to other nuclei for each isomer. The −21.6ppm hydride was a doublet of doublets with J_(HP)=52 and 57 Hz, andcorrelated to a ³¹P doublet of doublets at 94.8 and 75.3 ppm, withJ_(PH)=30 and J_(PP)=137 Hz. This hydride was also correlated to ¹³C{1H}NMR signals at 222.8 ppm for CO, 139.7 ppm for P—C(Ph) and 24.5 ppm forPC(iPr). Similarly, the −22.7 ppm hydride was a doublet of doublets withJ_(HP)=52 and 56 Hz, and correlated to a ³¹P doublet of doublets at 95.7and 75.0 ppm, with J_(PH)=21 and J_(PP)=136 Hz, and ¹³C{¹H} NMR signalsat 222.4 ppm for CO, 139.5 ppm for P—C(Ph) and 24.6 ppm for P—C(iPr).These species were quite similar, and were known to be mono-hydride,mono-carbonyl, iron PNP species, and thus were proposed to bemer-Fe(H)(OMe)(CO)(P—CH₂NH—P′), with N—H up and down, relative to thehydride for each isomer, respectively. The −21.6 ppm hydride wastherefore considered to be a kinetic product, and the −22.7 ppm hydride,a thermodynamic product.

A labeled alcohol, ¹³CH₃OH, was utilized to prepare 7b in order toverify the presence of the methoxide ligand. However, only free ¹³CH₃OHresonance was detected; this was likely due to a dynamic averaging ofthe methoxide and methanol resonances, since alkoxide ligands usuallyhydrogen-bond to an alcohol as hydrogen-bonded anions [ROH—OR]⁻[Clapham, S. E.; Guo, R.; Zimmer-De luliis, M.; Rasool, N.; Lough, A.;Morris, R. H. Organometallics 2006, 25, 5477-548681; Baratta, W.;Ballico, M.; Esposito, G.; Rigo, P. Chem. Eur. J. 2008, 14, 5588-5595].No enhancement of the Fe—₁₃CO signals were detected, and this showedthat isomers of 7b and any 9 present did not contain carbonyl ligandsderived from the alcohol [Zhang, J.; Gandelman, M.; Shimon, L. J. W.;Rozenberg, H.; Milstein, D. Organometallics 2004, 23, 4026-4033; Kloek,S. M.; Heinekey, D. M.; Goldberg, K. I. Organometallics 2006, 25,3007-3011].

Preformed solutions of 7b containing only the two hydride isomers at−21.6 and −22.7 ppm were tested for hydrogenation of acetophenone to1-phenylethanol under 5 atm H₂ pressure and 50° C., as optimized. Thesesolutions were active, converting 1000 equivalents of substrate in lessthan 15 minutes, but only in the presence of base.

A mixture of hydride complexes 8b without contamination by complex 9 wasalso tested for acetophenone hydrogenation at 50° C. and 25 atm H₂, inTHF, in the presence of base (KOtBu). Full conversion of acetophenone to1-phenylethanol occurred in 10 min with 0.2 mol % catalyst loading basedon initial amounts of 4b used to synthesize 8b.

¹H NMR hydride resonances of alkoxide species 7 and 8 (see [00134]) werefound to be similar in chemical shift and coupling constant to those ofFe{2,6-(PiPr₂CH₂)₂C₅H₃N}(H)(CO)(O^(i)Pr), as reported by Milstein et al.[Langer, R.; Leitus, G.; Ben-David, Y.; Milstein, D. Angew. Chem. Int.Ed. 2011, 50, 2120-2124]. The latter displayed a triplet hydrideresonance at −19.5 ppm with J_(HP)=53.5, however, and was thought to bea catalytically active form of precatalyst M2 (FIG. 1).

Chiral complex (S,S)-8d also existed as a mixture of iron mono-hydridecompounds with two major isomers (see [00134]). Again, large ²J_(PP)indicated trans phosphine coordination of the P—CH₂N—P′ ligand, whilethe large ²J_(HP) indicated cis hydride and phosphine ligands; however,phosphorus chemicals shifts of these minor compounds were onlyidentified via a ¹H—³¹P HMBC spectrum, implying they were in lowconcentration relative to the two major isomers of (S,S)-8d (see[00135]). Despite these complexes existing as a mixture of isomers, theywere active for ketone hydrogenation and, for (S,S)-8d, had goodenantioselectivity. For example, the mixture of hydrides catalyzedacetophenone hydrogenation to 1-phenylethanol with same e.e. andactivity as precatalyst (S,S)-4d activated in situ:

Example 4 Properties of Isomeric Alkoxide Hydrides (S,S)-7d asDetermined by DFT Analysis Experimental

Density Functional Theory Calculations on Simplified Models of (S,S)-7d:

DFT calculations were performed using Gaussian 09 [Frisch, M. J. et al.,2009, Gaussian 2009 Revision B. 2001]. M06 hybrid functional was usedfor all calculations [Zhao, Y.; Truhlar, D. Theor. Chem. Acc 2008, 120,215-241; Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157-167].All atoms were treated with a 6-31++G(d,p) basis set. A pruned (99,590)integration grid was used throughout (Grid=UltraFine). Optimizationswere performed in tetrahydrofuran using an integral equation formalismpolarizable continuum model (IEF-PCM) with radii and nonelectrostaticterms from a SMD solvation model [Marenich, A. V.; Cramer, C. J.;Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378-6396]. Full vibrationaland thermochemical analyses (1 atm, 298 K) were performed on optimizedstructures to obtain solvent-corrected free energies (G°) and enthalpies(H°). Optimized ground states were found to have zero imaginaryfrequencies. DFT gas phase calculations on full structures of (S,S)-7disomers were performed using GAMESS [GAMESS,http://www.msg.chem.iastate.edu/GAMESS/GAMESS.html; Schmidt, M. W.;Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J.H.; S. Koseki; Matsunaga, N.; Nguyen, K. A.; Su, S. J.; Windus, T. L.;Dupuis, M.; Montgomery, J. A. J. Comput. Chem., 1993, 14, 1347]. M06functional was used. Iron was treated with a LANL2DZ basis set with aneffective core potential [Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985,82, 270; ibid 284; ibid 299]. Atoms C, H, N, O and P were treated withthe 6-31 G basis set (see FIG. 10A).

Exemplary Density Functional Theory (Gaussian09M06/6-31++G(d,p)/(IEF-PCM)SMD(THF) Results for Most Stable SimplifiedModel of (S,S)-7d, F, with trans-(H)(OCH₃) and OCH₃ Adjacent to N—H (SeeFIGS. 12A and 12B):

Fe—H = 1.55, Fe—C = 1.69, Fe—O = 1.99, Fe—N = 2.07, Fe—P (PPh₂) = 2.23,Fe—P (PCy₂) = 2.24 E° = E^(o) = −4.27 P 1.053961924 6.8419492433.001709621 C 2.112154031 6.057059832 1.655127439 C −0.1314755174.622694905 4.234792446 C −2.510911004 4.592872861 5.064409655 H2.426760151 9.450264617 3.363165397 H −0.513552431 8.2374201621.609168999 H 3.542685842 14.108051198 8.158213181 N 1.5792312029.548526753 3.956127233 C 4.020971388 10.112900077 8.616009164 C−0.732072234 7.550466756 9.201333598 C 0.622573153 12.7329661699.220416398 C 1.358344348 7.059215394 10.301275969 H −2.2082979165.024187163 6.032741458 C 6.308706413 10.677484815 8.075675580 C1.282662711 10.483429811 6.233975702 H −1.092427635 2.9525250785.220458709 C 1.427423479 11.598272638 7.223600171 C 2.6924545506.284005362 5.414121334 C 2.533345382 12.464211505 7.212891431 H−0.124810677 12.831531480 10.002803214 H 0.230245672 10.1689011586.174806345 H −0.575293573 6.571164049 11.116848498 C 6.14501544810.135491359 6.800458434 H 1.838364116 14.358384166 9.952345008 H−1.904676308 7.394970802 3.097677257 H −3.392806127 3.9662171025.253820657 H 4.296950454 3.374467988 1.389501768 H −3.6673369996.346505084 4.499345468 Fe 2.032349009 7.718291014 4.814998601 C−1.631175201 6.598218861 3.804576828 O 3.269025490 5.3059867345.775479194 H 2.877977126 10.882799765 4.856535339 C 2.0539833677.643573434 9.241067400 H 0.227005259 5.068752524 5.177102714 H1.504582186 12.910024851 4.810851751 H 0.195635191 10.4753598582.607822865 H −1.679372746 3.256565097 3.582361224 H 2.3997776453.257435834 −0.209186769 C 1.184525455 12.042234859 4.225741576 H5.370002982 11.097770328 9.972546024 C 0.339537045 9.5260314873.139144681 C 2.380475633 5.349744532 −0.745465291 H 3.8580624033.787340121 −1.053335372 H 7.262849077 11.110647955 8.364287464 H1.514159973 12.194876356 3.190741624 C −0.037797625 8.1230022188.137867177 C −1.358428069 3.750774108 4.514950877 H 0.6024877477.653702417 5.408598050 C 3.121246359 4.086556369 −0.295724879 C1.782473779 10.769798411 4.814168289 P 2.249810873 8.8912733576.695933267 H 3.309193368 12.346682839 6.458873855 C 4.9216932299.573483210 6.430607001 C 3.855513305 9.562501974 7.339288178 H−0.514499036 9.379218857 3.812443905 H 0.869651268 6.732003195−0.014173961 H −3.227770183 5.293172401 3.151498033 C −0.4683412965.760781393 3.258999961 C 2.675864275 13.453434353 8.183397914 H−0.382943903 11.082428857 8.269229516 H 6.969671189 10.1458644866.093104575 H 1.908628588 6.629548233 11.133620889 H −0.7567584785.336771668 2.280736621 H −1.817519574 7.505123000 9.175651122 H3.265503735 4.885735479 3.092766949 C 1.387614708 5.8233153440.325170396 H 3.111901885 6.153331577 −0.928717448 H 4.7536750379.164129207 5.433648964 H 3.141093076 7.657459990 9.253604437 H2.020926695 3.973816480 2.227313997 H 1.196347818 8.6449960691.368266100 C −0.034518588 7.022495568 10.289717466 H 4.5869385635.064477980 0.962273508 C 0.475259838 11.750837820 8.242397182 H3.187046427 10.135827571 9.315534912 H 1.857369485 5.170593871−1.694656400 C 3.801376635 4.297244787 1.060379889 C 1.72327947813.590118159 9.193020773 H 0.692931465 4.008288964 3.849663540 C−2.855764372 5.725457917 4.095799522 H −1.291215856 7.0870150124.732892303 H 2.890843202 6.829108429 1.518998447 C 0.4399736228.395092981 2.126205099 C 5.247282376 10.665829246 8.982956931 H0.087416465 12.023352526 4.248480860 H 0.617010760 5.0475543930.469146723 C 1.360056275 8.191985291 8.159001506 H −0.5795636068.482624658 7.265580560 C 2.787269973 4.758527213 2.112557565 O3.648671068 8.266154852 3.782180906 C 4.813963366 7.4945980763.573375210 H 5.585630632 8.110393053 3.083926992 H 4.6461275956.613882898 2.924523300 H 5.241795145 7.110596395 4.517546369 ΔG_(solv)= −1598206.011 15 1.956664205 0.909695829 −0.244427984 1 −0.5092205651.018241026 1.400760050 1 1.516924320 3.338436518 −0.406725669 6−2.758857899 −0.127532876 −0.632552046 6 1.380740116 −1.867281559−0.702249595 1 −2.518394899 0.350204697 −1.595855710 26 0.339395174−0.575272720 −0.193269099 8 2.099002101 −2.710513509 −1.072723798 1−2.456950864 0.230115279 1.443448641 1 −4.197662030 1.7947847100.856773231 6 −3.163583671 2.057623477 0.609222203 1 −2.8000523472.717820372 1.405643202 1 0.117422387 −0.200247793 −1.689235889 6−2.322947642 0.797721267 0.506216071 1 1.213049317 2.6874585631.214442450 6 1.081374920 2.492690463 0.139639974 1 −3.1793096762.620701086 −0.333396500 8 0.445411592 −0.734136435 1.797850006 61.018279455 −1.856644551 2.337169064 1 0.978508684 −1.8382125463.445898040 1 2.094743176 −1.987198997 2.072645909 1 0.520308348−2.805466566 2.026860023 15 −1.607387837 −1.573108455 −0.480385441 1−2.289551679 −2.325577473 0.519504026 1 −1.940876118 −2.365161831−1.602561376 6 3.323052161 0.816087194 0.962922493 1 3.977803039−0.022537699 0.694999189 1 3.915153517 1.739733718 0.979734253 12.902196426 0.625412307 1.956558968 6 2.823621468 1.226597800−1.820515202 1 3.565565611 2.028463112 −1.717689829 1 3.3298103840.310025913 −2.147964220 1 2.091533261 1.503134072 −2.587783763 6−4.228087463 −0.509071248 −0.583421113 1 −4.502883974 −0.8852250430.412267417 1 −4.880477115 0.342469155 −0.813793825 1 −4.452865971−1.298501261 −1.310680075 6 −0.397334398 2.320362495 −0.170783598 1−0.575381175 2.277546711 −1.253720378 1 −0.956862260 3.1766508930.228133791 7 −0.845543094 1.047013706 0.432005262

Discussion

To support structural assignments for all alkoxides investigated,relative stabilities of said possible isomers were analyzed by DFTstudies. An initial study utilized a large basis set to treat simplifiedstructures of the methoxide isomers where phenyls on phosphorus werereplaced with hydrogen, and where cyclohexyl groups and backbone phenylswere replaced with methyl groups. This study was designed to revealelectronically preferred isomers with the mer-P—NH—P′ ligandstereochemistry that was established by NMR studies. Of the six possiblediastereomers, two isomers with hydride trans to carbonyl were found tobe high in energy, and thus too unstable to form (see paragraph[00154]). The isomer with a hydride trans to methoxide, and withmethoxide next to NH was most stable.

Steric effects on relative stabilities of the isomers was explored usinga gas phase calculation of the full structures treated with a smallerbasis set (see FIG. 10). There were four diastereomers that were low inenergy with isomer F being most stable. The methoxide oxygen was withinhydrogen-bonding distance of N—H group (O—H: 1.75 Å).

Example 5 Examination of the Mechanism of Activation via DihydrideComplexes

Described below are investigations using NMR spectroscopy and DFTcalculations to determine structures and/or characteristics of theactive hydride species present during catalyst activation and catalysisusing complexes 7b, or (S,S)-7d for ketone hydrogenation. Early in thesestudies, hydride resonances were observed in the ¹H NMR spectrum thatwere similar to previously known trans dihydride complexes [Langer, R.;Iron, M. A.; Konstantinovski, L.; Diskin-Posner, Y.; Leitus, G.;Ben-David, Y.; Milstein, D. Chem.—Eur. J. 2012, 18, 7196-7209], and so amore detailed examination of the mechanism of activation and catalysiswas pursued.

General Experimental

All procedures and manipulations were performed under an argon ornitrogen atmosphere using standard Schlenk-line and glove box techniquesunless stated otherwise. All solvents were degassed and dried usingstandard procedures prior to all manipulations and reactions unlessstated otherwise. Deuterated solvents were purchased from CambridgeIsotope Laboratories or Sigma Aldrich, degassed, and dried overactivated molecular sieves prior to use. All other reagents werepurchased from commercial sources and utilized without furtherpurification. NMR spectra were recorded at ambient temperature andpressure using a Varian Gemini 400 MHz spectrometer (400 MHz for 1H, 100MHz for ¹³C, 376 MHz for ¹⁹F, and 161 MHz for ³¹P), or an AgilentDD2-600 MHz spectrometer (600 MHz for ¹H, 151 MHz for ¹³C, 564 MHz for¹⁹F, and 243 MHz for ³¹P) unless stated otherwise. ¹H and ¹³C NMR weremeasured relative to partially deuterated solvent peaks, but werereported relative to tetramethylsilane (TMS). All ³¹P chemical shiftswere measured relative to 85% phosphoric acid as an external reference.Gas Chromatography was done on a Perkin Elmer Clarus 400 Chromatographequipped with a chiral column (CP chirasil-Dex CB 25 m×2.5 mm) todetermine substrate conversion and enantiopurity. Hydrogen gas was usedas the mobile phase, and the oven temperature was set at 130° C.Retention times for phenylethanol were 7.58 and 8.03 minutes, and foracetophenone, 4.56 minutes. All hydrogenation reactions were performedin a 50 mL stainless steel Parr Hydrogenation reactor at constanttemperatures and pressures. Temperature was maintained at 50° C. using aconstant temperature water bath and was purged of oxygen by flushing thereactor several times with 5 atm of H₂ (g).

Synthesis of Precatalysts: Iron precatalystsmer-trans-[Fe(Br)(CO)₂(P—CH═N—P′)][BF₄] were generated according to theforegoing procedure (see, for example, paragraph [00100]; as werealkoxide mono-hydride complexes FeH(OR′)(CO)(P—CH₂NH—P′) (7b and(S,S)-7d; see above).

Synthesis of trans-dihydride complex (11a-b/12a-b): Following thepreparation of alkoxide mono-hydride complexes FeH(OR′)(CO)(P—CH₂NH—P′)(7b and (S,S)-7d), a THF-d₈ solution (0.6 mL) of the respective alkoxidemono-hydride complexes was transferred to a Schlenk flask and reactedwith 1 atm hydrogen. After stirring under H₂ (g) for 5 minutes, base (˜8mg) was added in THF-d₈ (0.3 mL) under H_(2 (g)). Bases used includedKOtBu, NaOtBu, Ph-CH(OK)CH₃, and NaOMe; however, it was found that NaOMewas not an effective base, and KOtBu was primarily used throughouttesting. Upon addition of base, the bright orange solution rapidly turnsbright pink, then dark green/brown over 30 minutes. The solution wasthen injected into an NMR tube filled with H₂ (g) and the resonances dueto the dihydrides were observed using ³¹P and ¹H NMR spectroscopy.

NMR data for a mixture of trans-dihydride 11a and cis-dihydride 12a in a9:1 ratio. 11a: ³¹P{¹H} NMR (THF-d₈) 118.0 (d) and 95.8 ppm (d,J_(PP)(trans) 118 Hz). ¹H NMR (THF-d₈) −9.05 and −9.16 ppm (AB part ofABXY pattern, ²J_(HH)=J_(AB)=9.8 Hz, ²J_(PP)(trans)=J_(XY)=118.0 Hz, and²J_(HP)(cis) (J_(AX), J_(AY), J_(BX), J_(BY))=42.0, 42.0, 43.0, and 43.0Hz. 12a: ³¹P{¹H} NMR (THF-d₈) 114.5 (d) and 93.0 ppm (d, J_(PP) 90 Hz)¹HNMR (THF-d₈) −8.1 ppm (m) and −20.6 ppm (m).

NMR data for mixture of trans-dihydride 11b and cis-dihydride 12b in a1:1 ratio. 11b: ³¹P{¹H} NMR (THF-d₈) 110.2 (d) and 106.5 ppm (d, J_(PP)113.6 Hz). ¹H NMR (THF-d₈) −8.56 and −8.94 ppm (m, AB part of ABXYpattern). 12b: ³¹P{¹H} NMR (THF-d₈) 102.5 (d) and 109.7 ppm (d,J_(PP)(trans) 116.4 Hz). ¹H NMR (THF-d₈) −7.31 ppm (m) and −21.00 ppm(m).

To further remove excess base, benzene (3 mL) was added to the solutionin a vial in a nitrogen filled glovebox. The solution was then filteredthrough Celite and dried. To the dried solution, THF-d₈ (0.7 mL) wasadded and further stirred under H₂ (g) for 10 minutes and gave resolved¹H NMR spectra consistent with the analysis reported above.

Hydrogenation studies with preformed dihydride catalyst systems: Stocksolutions of acetophenone (0.3 mL, 2.6 mmol) in THF (7 mL) were injectedinto Parr reactors heated to 50° C. and pressurized to 5 atm H₂ (g)against a flow of hydrogen. 12-inch needles equipped with 1 mL syringeswere used to remove THF-d₈ solutions of the dihydrides 11a-b/12a-b fromrubber-septum capped NMR tubes prepared as outlined above, and injectedinto the reactors against a flow of hydrogen. At set times, smallamounts of sample were removed from the reactor using a needle andsyringe under a flow of hydrogen and injected into the gas chromatographfor analysis.

Hydrogenation studies with in-situ generated systems: In an argon filledglovebox, a vial was charged with [Fe(CO)2(Br)(PNP′)][BF4] (4b,(S,S)-4d) (5 mg, 0.006 mmol) and 3 mL THF. To this solution, LiAlH4 inTHF (0.05 mL of 1.0 M solution) was added, after which the solution'scolor immediately changed from pink to dark brown. After stirring for 5minutes, an alcohol (for example: 2-methyl-2-butanol) (0.5 mL) was addedand the solution was allowed to stir for an additional 10 minutes. Thesolution was then transferred to a syringe equipped with a 12-inchneedle. The same vial was then charged with acetophenone (0.35 mL, 3.0mmol) and 3 mL THF. The solution was taken up into the same syringe thatalready contained the precatalyst solution and stoppered. In a secondsyringe equipped with a 12-inch needle, a solution of base (0.08 mmol ofeither KOtBu, NaOtBu, or NaOMe) in 3 mL THF was taken up and stoppered.Both syringes were removed from a glovebox and injected into Parrreactors heated to 50° C. and pressurized to 5 atm H₂ (g) against a flowof hydrogen. At set times, small amounts of sample were removed from thereactor using a needle and syringe under a flow of hydrogen, andinjected into the gas chromatograph for analysis. As discussed below,complete conversion was observed in 15 minutes using either KO^(t)Bu orNaO^(t)Bu. Using NaOMe, however, no conversion to product alcohol wasobserved after 3 hours.

For experiments wherein cryptand was added to the catalyst system, athird syringe equipped with a 12-inch needle was charged with2,2,2-cryptand (36 mg, 0.095 mmol) in 0.8 mL THF and stoppered. Thissolution was injected into the Parr reactor 2 minutes after the additionof the catalyst/acetophenone and base solutions. As discussed below,conversion to alcohol was complete in 15 minutes, as with the systemswithout cryptand; this suggested no interaction with cations duringcatalysis.

Computational Details: Density functional theory calculations wereperformed using Gaussian09 package [Frisch, M. J. 2010; Vol. Gaussian 09Revision B.01] and M11-L hybrid functional [Peverati, R.; Truhlar, D. G.J. Phys. Chem. Lett. 2011, 3, 117-124; Kulkarni, A. D.; Truhlar, D. G.J. Chem. Theory Comput. 2011, 7, 2325; Zhao, Y.; Truhlar, D. Theor.Chem. Acc. 2008, 120, 215; d) Zhao, Y.; Truhlar, D. G. J. Chem. Phys.2006, 125, 194101]. Functionals used for EDS calculation are provided inTable 3. All atoms were treated with a 6-31++G(d,p) basis set, whichincluded diffuse basis functions, except for EDS calculations where a6-31 G basis set was used [Clark, T.; Chandrasekhar, J.; Spitznagel, G.W.; Schleyer, P. V. R. J. Comput. Chem. 1983, 4, 294; Lynch, B. J.;Zhao, Y.; Truhlar, D. G. J. Phys. Chem. A 2003, 107, 1384; Frisch, M.J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265]. Apruned (99,590) integration grid was used throughout (Grid=UltraFine).Optimizations were performed using an integral equation formalismpolarizable continuum model (IEF-PCM) [Tomasi, J.; Mennucci, B.; Cammi,R. Chem. Rev. 2005, 105, 2999; Tomasi, J.; Mennucci, B.; Cancès, E. J.Mol. Struct. (THEOCHEM) 1999, 464, 211] with radii and nonelectrostaticterms from a SMD solvation model in THF [Marenich, A. V.; Cramer, C. J.;Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378]. Ground states wereconnected to their transition states by performing intrinsic reactioncoordinate (IRC) calculations [Fukui, K. Acc. Chem. Res. 1981, 14, 363].Complex 13a and 16a were calculated in their singlet and triplet statesfor comparison (FIG. 13). Stationary points were characterized bynormal-mode analysis. Full vibrational and thermochemical analyses (1atm, 298 K) were performed on optimized structures to obtainsolvent-corrected free energies (G°) and enthalpies (H°). Optimizedground states were found to have zero imaginary frequencies, whiletransition states were found to have one imaginary frequency.Three-dimensional visualizations of calculated structures were generatedby ChemCraft.

Identification of Dihydride Species by NMR Spectroscopy

In view of the foregoing working examples, an objective was to identifywhat iron-containing species were formed when the precursor alkoxidecomplexes were activated with base to produce catalyst solutions forketone hydrogenation (for example, see above scheme). Reaction ofcomplex 7b with hydrogen in THF-d₈ without base yielded no change after24 h. Addition of base (KOtBu or NaOMe) to 7b in the presence ofhydrogen resulted in consumption of the starting material and formationof a new trans-dihydride species 11a, with ³¹P{¹H} NMR doublets at 118.0and 95.8 ppm with a J_(PP) 118 Hz, as well as with ¹H NMR hydridesignals centred at −9.10 ppm. In addition, there was a smaller amount(10%) of a cis-dihydride isomer 12a with doublets in its ³¹P{¹H} NMRspectrum at 114.5 and 93.0 ppm with J_(PP) 90 Hz, along with complextriplets in its ¹H NMR spectrum at −8.1 and −20.6 ppm which correlatedby ³¹P-¹H HMBC (see below scheme).

The trans-dihydride signals of 11a were similar to those of thecorresponding isomers of dihydride J reported by Milstein and coworkers(see FIG. 14; Langer, R.; Iron, M. A.; Konstantinovski, L.;Diskin-Posner, Y.; Leitus, G.; Ben-David, Y.; Milstein, D. Chem.—Eur. J.2012, 18, 7196-7209], with the difference being that the hereindescribed trans complex had inequivalent hydrides. Selective decouplingof both the 118.0 and 95.8 ppm ³¹P signals associated with 11a resolvedthese signals into two hydride doublets at −9.05 and −9.16 ppm withJ_(HH) 9.8 Hz. The trans-hydrides were diastereotopic due to thepresence of the amine group. Complexes with inequivalenttrans-dihydrides are considered rare: trans-FeH₂(meso-tetraphos) hasJ_(HH) 18 Hz [Bautista, M. T.; Earl, K. A.; Maltby, P. A.; Morris, R. H.J. Am. Chem. Soc. 1988, 110, 4056-4057; Bautista, M. T.; Earl, K. A.;Maltby, P. A.; Morris, R. H.; Schweitzer, C. T. Can. J. Chem. 1994, 72,547-560], while cis dihydrides usually have J_(HH) in a range of 13-21Hz [Lagaditis, P. O.; Sues, P. E.; Sonnenberg, J. F.; Wan, K. Y.; Lough,A. J.; Morris, R. H. J. Am. Chem. Soc. 2014, 136, 1367-1380; Langer, R.;Iron, M. A.; Konstantinovski, L.; Diskin-Posner, Y.; Leitus, G.;Ben-David, Y.; Milstein, D. Chem.—Eur. J. 2012, 18, 7196-7209; Trovitch,R. J.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2006, 45, 7252-7260;Gusev, D. G.; Hubener, R.; Burger, P.; Orama, O.; Berke, H. J. Am. Chem.Soc. 1997, 119, 3716-3731; Schott, D.; Callaghan, P.; Dunne, J.;Duckett, S. B.; Godard, C.; Goicoechea, J. M.; Harvey, J. N.; Lowe, J.P.; Mawby, R. J.; Müller, G.; Perutz, R. N.; Poli, R.; Whittlesey, M. K.Dalton Trans. 2004, 3218-3224]; and, complex M (FIG. 14) in C₆D₆ hadJ_(HH) 9.6 [Bornschein, C.; Werkmeister, S.; Wendt, B.; Jiao, H.;Alberico, E.; Baumann, W.; Junge, H.; Junge, K.; Beller, M. Nat. Commun.2014, 5, doi 10.1038/ncomms5111; Chakraborty, S.; Brennessel, W. W.;Jones, W. D. J. Am. Chem. Soc. 2014, 136, 8564-8567]. The 118 Hz J_(PP)of 11a was of similar magnitude to that of 7b (J_(PP) 136 Hz), which hadtrans phosphorus nuclei in a mer P—N—P′ configuration on the metalcenter [Lagaditis, P. O.; Sues, P. E.; Sonnenberg, J. F.; Wan, K. Y.;Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2014, 136, 1367-1380],thus indicating that the ligand in 11a was also in a mer configuration,as shown in the above scheme. Simulation of the hydride's resonancepattern, using predicting software MestReNova 8, indicated that J_(PH)coupling to the PiPr₂ and PPh₂ groups were 42 Hz, somewhat lower thanthe 50-60 Hz splitting observed for complexes 7b and (S,S)-7d butsimilar to the 40.3 Hz observed for dihydride J [Langer, R.; Iron, M.A.; Konstantinovski, L.; Diskin-Posner, Y.; Leitus, G.; Ben-David, Y.;Milstein, D. Chem.—Eur. J. 2012, 18, 7196-7209].

Reaction of chiral system (S,S)-7d in THF-d₈ with KOtBu under dihydrogengenerated trans and cis dihydride isomers (S,S)-11b and (S,S)-12b,respectively, in an approximate 1:1 ratio. Dihydride (S,S)-11b had twodoublets in its ³¹P NMR spectrum at 110.2 and 106.5 ppm, with²J_(PP)=113.6 Hz for the trans phosphorus nuclei in a mer-(P—N—P′)ligand. Two sets of hydride multiplet resonances were evident in its ¹HNMR spectrum at −8.56 and −8.94 ppm with ²J_(HH)=9.3 Hz; thesecorrelated with the 110.2 and 106.5 ppm ³¹P signals through ¹H—³¹P HMBC.³¹P NMR spectrum of cis-(S,S)-12b had two doublets at 102.5 and 109.7ppm with ²J_(PP)=116.4 Hz, with hydride multiplets appearing at −7.31and −21.00 ppm with ²J_(HH)=15.5 Hz in the ¹H NMR spectrum, similar tothe cis-dihydride isomer of M [FIG. 14, Bornschein, C.; Werkmeister, S.;Wendt, B.; Jiao, H.; Alberico, E.; Baumann, W.; Junge, H.; Junge, K.;Beller, M. Nat. Commun. 2014, 5, doi 10.1038/ncomms5111; Chakraborty,S.; Brennessel, W. W.; Jones, W. D. J. Am. Chem. Soc. 2014, 136,8564-8567].

Trans dihydrides J [Langer, R.; Leitus, G.; Ben-David, Y.; Milstein, D.Angew. Chem. Int. Ed. 2011, 50, 2120-2124] and M [Bielinski, E. A.;Lagaditis, P. O.; Zhang, Y. Y.; Mercado, B. Q.; Wurtele, C.;Bernskoetter, W. H.; Hazari, N.; Schneider, S. J. Am. Chem. Soc. 2014,136, 10234-10237] of FIG. 14 have been reported to interconvert with cisdihydrides on a NMR timescale at room temperature. Exchange was notdetected in NOESY spectra between 11a and 12b. However, it was observedthat the dihydrides always maintained the same ratio of concentration,suggesting that they may interconvert on a slower timescale. Othernon-rigid, six-coordinate iron dihydride complexes have also been knownto undergo dynamic isomerization [Langer, R.; Iron, M. A.;Konstantinovski, L.; Diskin-Posner, Y.; Leitus, G.; Ben-David, Y.;Milstein, D. Chem.—Eur. J. 2012, 18, 7196-7209; Gerlach, D. H.; Peet, W.G.; Muetterties, E. L. J. Am. Chem. Soc. 1972, 94, 4545-4549; Meakin,P.; Muetterties, E. L.; Jesson, J. P. J. Am. Chem. Soc. 1973, 95,75-88.].

Reaction of 7b with KOtBu in THF-d₈ under argon rapidly produceddicarbonyl complex 9 and other species (see FIG. 15). This contrastedwith the reaction of bromohydrido complex K, or borohydrido complex L,which reacted with KOtBu under Ar to give amidohydrido complex N, shownin FIG. 14 [Bornschein, C.; Werkmeister, S.; Wendt, B.; Jiao, H.;Alberico, E.; Baumann, W.; Junge, H.; Junge, K.; Belier, M. Nat. Commun.2014, 5, doi 10.1038/ncomms5111; Chakraborty, S.; Brennessel, W. W.;Jones, W. D. J. Am. Chem. Soc. 2014, 136, 8564-8567; Bielinski, E. A.;Lagaditis, P. O.; Zhang, Y. Y.; Mercado, B. Q.; Wurtele, C.;Bernskoetter, W. H.; Hazari, N.; Schneider, S. J. Am. Chem. Soc. 2014,136, 10234-10237]. Structure of N was elucidated by single crystal X-raydiffraction [Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am.Chem. Soc. 2014, 136, 8564-8567]. The two PiPr₂ groups on the ligand inN appeared to be more effective at stabilizing an amido complex of thistype, with respect to formation of iron(0) species, than the combinationof PiPr₂ and PPh₂ on the unobserved amido species 13a (FIG. 16). It wasconsidered that the smaller PPh₂ group may allow intermolecular carbonylexchange to occur, ultimately leading to iron(0) dicarbonyl 9 and otherspecies by a reductive elimination process, or direct reductiveelimination of the hydrido and amido donors in 13a (discussed below).

Catalytic Testing

To determine whether dihydride complexes 11a/12a were the active speciesof the catalysis described herein, the complexes were tested withacetophenone to gauge conversion to 1-phenylethanol under H₂.Consequently, THF-d₈ solutions of the dihydrides 11a/12a were injectedinto a pressurized Parr reactor set at 50° C. and 5 bar H₂, along with0.3 mL acetophenone (catalyst:substrate˜1:500); complete conversion to1-phenylethanol was observed in less than 15 minutes. The insitu-generated catalysts from 4b (described above) achieved completeconversion in 15 minutes as well. Without wishing to be bound by theory,it was considered that this suggested that dihydride species 11a/12awere the active catalyst, or an entry point into the catalytic cycle.Under the same conditions, the same conversion was observed forsolutions of chiral dihydrides (S,S)-(S,S)-11b/12b with an e.e. of 83%(S)-1-phenylethanol, as was reported for the in situ-generated system.Thus, the dihydrides observed by NMR are the same in theenantioselective catalytic system. In other words, dihydrides(S,S)-11b/12b, observable by NMR, showed the same activity andselectivity as the in-situ system generated from (S,S)-4d, indicatingthat it was or directly lead into an active species in catalysis. It wasobserved that, if 0.05 mL of acetophenone was injected into a J-YoungNMR tube containing dihydrides 11a/12a, and maintained a pressure of 1atm H₂ at 30° C., the dihydride resonances disappeared and a lowconcentration of 1-phenyethanol was detected by GC. The catalyst systemwas active at 30° C., but pressures higher than 1 atm were required forcatalyst turnover.

As described above, generation of the dihydride appeared to requirebase. To further test this, solutions containing the dihydride species11a/12a were dried and any residue re-dissolved in benzene, and allexcess base was filtered off; the dihydride solution was then directlyinjected into a reactor pressurized to 5 atm H₂. Minimal catalysis wasobserved, however, which suggested that base was required to generatethe dihydride complexes, and for catalytic turnover. It was postulatedthat the observed requirement for base was a consequence of the alcoholbeing in excess during the catalysis, which may facilitate the ironcomplex being converted back to an alkoxide complex like 7b. Withoutwishing to be bound by theory, it was considered that the excess basewas required to return the alkoxide complexes to the catalytic cycle, asdiscussed further below. This proposal was consistent with systemscontaining complexes 8 (R′″=tAmyl) generating more active catalystsystems than systems containing complexes 7 (R′″=Me), which form morestable alkoxides with less steric hindrance.

Without wishing to be bound by theory, another explanation wasconsidered for the observed requirement for the presence of base duringcatalysis: that anionic amido species stabilized by potassium wereinvolved in the catalytic cycle. The presence of such species have beenproposed for Noyori ruthenium diamine catalyst systems during ketonehydrogenation [Hartmann, R.; Chen, P. Angew. Chem. Int. Ed. 2001, 40,3581-3585; John, J. M.; Takebayashi, S.; Dabral, N.; Miskolzie, M.;Bergens, S. H. J. Am. Chem. Soc. 2013, 135, 8578-8584; Dub, P. A.;Henson, N. J.; Martin, R. L.; Gordon, J. C. J. Am. Chem. Soc. 2014, 136,3505-3521]. It was found that addition of 1.1 equivalents per potassiumion of 2,2,2-cryptand to the activated catalyst mixture of7b/KOtBu/acetophenone/H₂ (5 atm) had no effect on rate of catalysis. Asformation of a stable [K(2,2,2-cryptand)]⁺ complex should have preventedpotassium from engaging in bonding with an amido ligand, alkoxide baseor ketone substrate, this ruled out potassium-specific mechanisms asproposed for the Noyori system.

Postulated Mechanism for Ketone Hydrogenation

Observation of trans-dihydridoamine complexes 11a and (S,S)-11b incatalytically active solutions led to a proposed hydrogenation pathway[Alberico, E.; Sponholz, P.; Cordes, C.; Nielsen, M.; Drexler, H.-J.;Baumann, W.; Junge, H.; Beller, M. Angew. Chem. Int. Ed. 2013, 125,14412-14416; Chakraborty, S.; Dai, H.; Bhattacharya, P.; Fairweather, N.T.; Gibson, M. S.; Krause, J. A.; Guan, H. J. Am. Chem. Soc. 2014, 136,7869-7872; Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am. Chem.Soc. 2014, 136, 8564-8567; Abdur-Rashid, K.; Clapham, S. E.; Hadzovic,A.; Harvey, J. N.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2002,124, 15104-15118; Abdur-Rashid, K.; Lough, A. J.; Morris, R. H.Organometallics 2000, 19, 2655-2657; Clapham, S. E.; Hadzovic, A.;Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201-2237; Hadzovic, A.;Song, D.; MacLaughlin, C. M.; Morris, R. H. Organometallics 2007, 26,5987-5999; e) Noyori, R. Angew. Chem. Int. Ed. 2002, 41, 2008-2022;Bertoli, M.; Choualeb, A.; Lough, A. J.; Moore, B.; Spasyuk, D.; Gusev,D. G. Organometallics 2011, 30, 3479-3482], as shown in FIG. 16. Themechanism was also supported by DFT calculations, discussed below.

Base was required to generate an intermediate that reacted withdihydrogen, thought to be hydrido amido complex 13a, analogous tostructure N determined by DFT calculations, shown in FIG. 14. Thestructure of a similar complex, FeH(CO)(PiPr₂CH₂CH₂NCH₂CH₂PiPr₂), hasrecently been determined by single crystal X-ray diffraction[Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am. Chem. Soc.2014, 136, 8564-8567]. It was proposed that base bound the alcohol thatwas released, forming a hydrogen-bonded alkoxide adduct of the formK[RO—HOR′] in a low dielectric constant solvent (THF). According to theDFT calculations, the starting alkoxide complex 7b was too stablewithout addition of base. Addition of dihydrogen trans to the hydride incomplex 13a, in a similar fashion to 16-electron ruthenium hydrido amidocomplexes RuH(NH-L)(PR₃)₂ [Abdur-Rashid, K.; Lough, A. J.; Morris, R. H.Organometallics 2000, 19, 2655-2657; Clapham, S. E.; Hadzovic, A.;Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201-2237; Hadzovic, A.;Song, D.; MacLaughlin, C. M.; Morris, R. H. Organometallics 2007, 26,5987-5999], was found to lead to trans-dihydride complex 11a. Unlikewith the ruthenium systems, it was found that dihydrogen can also addtrans to the carbonyl in 13a to produce a cis dihydride 12a.

It was then proposed that the trans-dihydride complex transfers a protonfrom the ligand's nitrogen, and a hydride from the iron to a ketone'scarbonyl group to give an alcohol product, thus regenerating the amidocomplex 13a. Trans-dihydrides on ruthenium [Abdur-Rashid, K.; Clapham,S. E.; Hadzovic, A.; Harvey, J. N.; Lough, A. J.; Morris, R. H. J. Am.Chem. Soc. 2002, 124, 15104-15118; Abdur-Rashid, K.; Lough, A. J.;Morris, R. H. Organometallics 2000, 19, 2655-2657; Clapham, S. E.;Hadzovic, A.; Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201-2237;Hadzovic, A.; Song, D.; MacLaughlin, C. M.; Morris, R. H.Organometallics 2007, 26, 5987-5999; Abbel, R.; Abdur-Rashid, K.; Faatz,M.; Hadzovic, A.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2005,127, 1870-1882; Hamilton, R. J.; Bergens, S. H. J. Am. Chem. Soc. 2008,130, 11979-11987; Takebayashi, S.; Bergens, S. H. Organometallics 2009,28, 2349-2351; Takebayashi, S.; John, J. M.; Bergens, S. H. J. Am. Chem.Soc. 2010, 132, 12832-12834] and osmium [Bertoli, M.; Choualeb, A.;Lough, A. J.; Moore, B.; Spasyuk, D.; Gusev, D. G. Organometallics 2011,30, 3479-3482] have been known to rapidly attack ketones, esters andimides in their outer coordination sphere; wherein the metal transferreda hydride to the carbonyl carbon, and the amine transferred a proton tothe carbonyl oxygen in a metal-ligand bifunctional process, sometimes ina stepwise fashion [Noyori, R. Angew. Chem. Int. Ed. 2002, 41,2008-2022; Bertoli, M.; Choualeb, A.; Lough, A. J.; Moore, B.; Spasyuk,D.; Gusev, D. G. Organometallics 2011, 30, 3479-3482]. Iron hydridecomplexes have been proposed to do the same bifunctional transfer[Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282-2291; Sues, P. E.;Demmans, K. Z.; Morris, R. H. Dalton Trans. 2014, 43, 7650-7667;Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Angew. Chem. Int.Ed. 2008, 47, 940-943; Prokopchuk, D. E.; Morris, R. H. Organometallics2012, 31, 7375-7385; Zuo, W.; Tauer, S.; Prokopchuk, D. E.; Morris, R.H. Organometallics 2014, doi: 10.1021/om500479q].

It was further postulated that the product alcohol might also react withthe amido complex 13a, in a manner similar to the methanol, as shown inFIG. 16. It was considered that thus trapping the amido complex 13a bydihydrogen or alcohol may protect it from a reductive eliminationpathway, or a bimolecular carbonyl exchange pathway that may lead tocatalytic degradation into dicarbonyl 9 and other species.

DFT Studies

Structural optimization of each intermediate and transition stateinvolved in the catalytic cycle using 7b was done using a functionalM11-L with a 6-31++G(d,p) basis set with a THF solvation model.Important structures and transition states are shown in FIG. 17; areaction coordinate diagram for the catalytic process is shown in FIG.18. All energies were referenced to that of dihydride 11a. The dihydridehad a distorted octahedral structure with Fe—H distances of 1.55 Å (Htrans to H), longer than 1.54 (C trans to H) and 1.53 Å (N trans to H)bond lengths calculated for 12a due to higher trans influence of thehydride ligand compared to carbonyl and amine ligands, respectively[Abdur-Rashid, K.; Clapham, S. E.; Hadzovic, A.; Harvey, J. N.; Lough,A. J.; Morris, R. H. J. Am. Chem. Soc. 2002, 124, 15104-15118; Abbel,R.; Abdur-Rashid, K.; Faatz, M.; Hadzovic, A.; Lough, A. J.; Morris, R.H. J. Am. Chem. Soc. 2005, 127, 1870-1882; Bau, R.; Chiang, M. Y.; Ho,D. M.; Gibbins, S. G.; Emge, T. J.; Koetzle, T. F. Inorg. Chem. 1984,23, 2823; Rybtchinski, B.; Ben-David, Y.; Milstein, D. Organometallics1997, 16, 3786-3793; Lin, Z.; Hall, M. B. Coord. Chem. Rev. 1994,135/136, 845-879]. These values were consistent with those determined inother iron hydride structures using single crystal neutron diffraction[Bau, R.; Chiang, M. Y.; Ho, D. M.; Gibbins, S. G.; Emge, T. J.;Koetzle, T. F. Inorg. Chem. 1984, 23, 2823; Ho, N. N.; Bau, R.; Mason,S. A. J. Organomet. Chem. 2003, 676, 85; Ricci, J. S.; Koetzle, T. F.;Bautista, M. T.; Hofstede, T. M.; Morris, R. H.; Sawyer, J. F. J. Am.Chem. Soc. 1989, 111, 8823; Sluys, L. S. V. D.; Eckert, J.; Eisenstein,O.; Hall, J. H.; Huffman, J. C.; Jackson, S. A.; Koetzle, T. F.; Kubas,G. J.; Vergamini, P. J.; Caulton, K. G. J. Am. Chem. Soc. 1990, 112,4831]. Following the reaction as depicted in FIG. 18, the dihydrideformed a weak adduct with acetophenone (17a) via an NH—O hydrogen bondwith a modest increase in energy to G°=5.1 kcal/mol.

Transfer of the metal hydride to the ketone in transition statestructure TS17a,18a (FIGS. 17 and 18, full structure calculation) had afree energy of activation of 20.9 kcal/mol, consistent with a processoccurring in a temperature range of 25-50° C. It was considered thatsaid energy barrier, due to sterics, was higher than 13.5 kcal/molcalculated for TS_(P,Q) of a simplified system, wherein phenyl groupswere replaced by hydrogens and isopropyl groups with methyls (see FIG.19). The calculated 20.9 kcal/mol barrier was similar to that of thedihydrogen splitting step (TS21a,11a of FIG. 18), and was expected to berate determining for bulkier ketone substrates. This transfer producedan alkoxide adduct (Q, FIG. 19 of the simplified system, not yet locatedfor the full system), which received a proton from the NH group toproduce the phenylethanol adduct 19a (see similar simplified structuresQ and R in FIG. 20). A proton transfer transition state was locatedusing a simplified ligand structure (FIG. 20), but not for the fullstructure. A slow hydride transfer to a ketone followed by a fast protontransfer step [Prokopchuk, D. E.; Morris, R. H. Organometallics 2012,31, 7375-7385; Zweifel, T.; Naubron, J.-V.; Bittner, T.; Ott, T.;Grützmacher, H. Angew. Chem. Int. Ed. 2008, 47, 3245-3249; Zweifel, T.;Naubron, J.-V.; Gritzmacher, H. Angew. Chem. Int. Ed. 2009, 48, 559-563;Bertoli, M.; Choualeb, A.; Gusev, D. G.; Lough, A. J.; Major, Q.; Moore,B. Dalton Trans. 2011, 40, 8941-8949], or an asynchronous hydride-protontransfer [Guo, X.; Tang, Y.; Zhang, X.; Lei, M. J. Phys. Chem. A. 2011,115, 12321-12330; b) Chen, Y.; Liu, S.; Lei, M. J. Phys. Chem. C 2008,112, 13524-13527] has been considered common for similar metal-amidoketone hydrogenation catalysts (Fe, Ru or Os).

Liberation of alcohol product from 19a generated the diamagnetic ironhydridoamido complex 13a (G″=−0.3 kcal/mol with acetophenone and H₂).This amido complex was almost square pyramidal with a hydride in anapical position, and a planar amido nitrogen (FIG. 17) having a C—Fe—Nangle of 168°, a C—Fe—H angle of 90° an N—Fe—H angle of 103°; the sum ofthe angles around nitrogen was 358°. By contrast, the crystal structureof hydridoamido complex N (FIG. 14) had a distortion towards trigonalbipyramidal with corresponding angles of 156°, 83° and 90° with a sumaround N of 358°. Compared to a Fe—N(amine) distance of 2.08 Å fordihydride 11a, 13a had a much shorter Fe—N(amido) distance of 1.87 Å,which was comparable to the Fe—N(amido) bond of 1.86 Å in N. Thestructure of a distorted 13a (S=1) trigonal bipyramidal hydridoamidostructure in a triplet state was also optimized, and found to be 20.6kcal/mol higher in energy than diamagnetic 13a (FIG. 13). It wasconsidered that a possible route to the dicarbonyl decomposition product9 may have been via reductive elimination of 13a to an iron(0)monocarbonylamine complex Fe(CO)(P—NH—P′) 16a. However this would havehad to involve a spin state change, as the resulting iron(0) complex 16awas calculated to be more stable as a distorted tetrahedral S=1structure by 8 kcal/mol, than a distorted square planar S=0 structure(FIG. 13).

Reaction of hydridoamido complex 13a with dihydrogen led to an end-onadduct of dihydrogen with amido complex 20a having an energy of G°=4.5kcal/mol, and then a higher energy η²-H₂ structure, 21a (G°=11.0kcal/mol). Splitting of the coordinated hydrogen across the metal andamido nitrogen, TS21a, 11a (FIG. 17, 19, G°^(†)=20.3 kcal/mol) wascalculated to be as energetically demanding as the hydride attack on theketone; as such, it was considered that it could be rate-determiningunder certain conditions. It was also considered that this could explainan experimental finding that increased pressure increased rate ofhydrogenation of certain substrates such as 2-acetylpyridine; however,the system was complicated by many equilibria such as with alcohol, togive inactive alkoxide complexes, and with enolizable ketones, to giveinactive enolate complexes, which influenced the amount of activecatalyst and therefore rate. Further, product phenylethanol and its rolein shuttling a proton in the dihydrogen splitting step, from thedihydrogen to the amido nitrogen via the alcohol, was examined[Hadzovic, A.; Song, D.; MacLaughlin, C. M.; Morris, R. H.Organometallics 2007, 26, 5987-5999; Ito, M.; Hirakawa, M.; Murata, K.;Ikariya, T. Organometallics 2001, 20, 379-381; Hasanayn, F.; Morris, R.H. Inorg. Chem. 2012, 51, 10808-10818]; however, it was found that thetransition state (TS22a,23a, 20.3 kcal/mol, FIG. 21) was similar inenergy to the transition state (TS21a,11a, 20.3 kcal/mol, FIG. 18) ofthe dihydrogen splitting mechanism, without the alcohol serving as aproton shuttle from dihydrogen to the amido nitrogen.

Alkoxide complex 7b was calculated to be 9.6 kcal/mol lower in energythan hydridoamido complex 13a in THF, in the absence of base (FIG. 16).When KOtBu was added to 7b, the system was calculated to be 5.5 kcal/molmore stable than the hydridoamido complex 13a plus the stronglyhydrogen-bonded salt K[tBuOH—OR′] in THF (FIG. 17). In view of theforegoing, it was postulated therefore that the presence of basedestabilized the alkoxide by approximately 4 kcal/mol, as proposedabove. It was considered that this observed effect may be greater due toKOtBu existing as aggregates in solvents of low dielectric constant,such as THF [Chisholm, M. H.; Drake, S. R.; Naiini, A. A.; Streib, W. E.Polyhedron 1991, 10, 337-345; Song, D.; Morris, R. H. Organometallics2004, 23, 4406-4413; Evans, W. J.; Sollberger, M. S.; Ziller, J. W. J.Am. Chem. Soc. 1993, 115, 4120-4127], and that breaking up theseaggregates by way of alcohol hydrogen-bonding to an alkoxide would beentropically favorable. Similarly, the 1-phenylethanol product of thecatalytic reaction can form an alkoxide Fe(CO)(P—NH—P′)(OCHMePh) (FIG.16, 14 a), which was calculated to have relative energies of −7.1kcal/mol without base, and −2.5 kcal/mol with KOtBu. This suggestedthat, during the catalytic cycle, some active species may be revertedback to the alkoxide complex, while added potassium or sodium alkoxidemay help to reverse this process.

Also investigated was formation of cis-dihydride intermediate 12a, whichwas observed in minor concentration during the above-described NMRstudies. Dihydrogen species 15a, which led to 12a, as shown in FIG. 18,was calculated to be 3.5 kcal/mol higher in energy than 21a, thecorresponding dihydrogen species that led to 11a (FIG. 18). Similarly,15a led to a heterolytic hydrogen splitting transition state TS15a, 12athat had a similar energy to TS21a, 11a (G″^(†)=20.3 kcal/mol) toproduce 12a (G″^(†)=3.7 kcal/mol), which had a higher energy than 11a.This was consistent with an experimental observation that thecis-dihydride compound 12a was in smaller quantity compared to thetrans-dihydride compound 11a.

In addition to the theoretical catalytic mechanistic study describedabove, full structure DFT calculations were performed using a smallerbasis set (6-31 G) on an enantiodetermining step (EDS) of complex(S,S)-11b, with acetophenone as test substrate. This approach has beenused successfully in other DFT investigations of outer sphere ketoneasymmetric hydrogenation [Dub, P. A.; Henson, N. J.; Martin, R. L.;Gordon, J. C. J. Am. Chem. Soc. 2014, 136, 3505-3521; Chen, H.-Y. T.; DiTommaso, D.; Hogarth, G.; Catlow, C. R. A. Dalton Trans. 2012, 41,1867-1877; Feng, R.; Xiao, A.; Zhang, X.; Tang, Y.; Lei, M. DaltonTrans. 2013, 42, 2130-2145]. The hydride transfer step from iron to theketone carbonyl was identified as the EDS (FIG. 22), and activationbarriers for the S- and R-isomer were calculated to be 19.7 kcal/mol,and 22.1 kcal/mol, respectively (FIG. 24). This 2.4 kcal/mol differencerepresents an enantiomeric excess of 97% in the S-isomer, as calculatedusing transition state theory, which was considered to be fairlyconsistent with the experimental evidence, 89% ee at 25° C. (see above),given that a crude basis set was used. A consistent bias toward Salcohol production was observed when other functionals were employed,although predicted ee values varied 93-99% (see Table 3). After thehydride transfer, proton transfer and alcohol liberation completedconversion of prochiral acetophenone into (S)-1-phenylethanol.

Three factors were identified that contributed to the catalyst'senantioselectivity. First was size difference between the largercyclohexyl substituents and the smaller phenyl substituents on thephosphine ligands. As depicted in FIG. 22, the more energeticallyfavoured EDSs had the substrate's smaller methyl group oriented over thebulkier cyclohexyl groups; the more energetically demanding EDS_(R), hadthe ketone's phenyl group positioned there. Secondly, in both scenarios,the phenyl and cyclohexyl substituents from the phosphine were visiblycompressed away from the ketone, with the degree of compression smallerfor the methyl group on the ketone than the phenyl. The phenyl groups onthe ligand appeared to be more compressible because they could stackagainst each other. Thirdly, hydrogen-bonding strength, which wasconsidered to have also contributed to the overall energy gap betweenEDSs and EDS_(R). Calculated O—H bond distance between the oxygen in theketo-substrate, and the proton in the ligand's NH moiety in EDSs was1.86 Å, while in EDS_(R), it was 2.12 Å; this signalled a strongerhydrogen bond in the former structure.

CONCLUSIONS

An effective synthesis templated by iron(II) to make unsymmetricalP—N—P′ ligands by condensation of phosphine-amines withphosphine-aldehydes, generated from phosphonium dimers, has beendeveloped. A mixture of trans- and cis-iron complexes,Fe(CO)(Br)₂(P—CH═N—P′), 2 and 3, were initially synthesized from aone-pot reaction with phosphonium dimers (1a-c), KOtBu, FeBr₂ andPhP₂C₂H₄NH₂ in THF under a CO(g) atmosphere. Upon addition of AgBF₄ tosaid complexes under a CO atmosphere, new complexestrans-[Fe(CO)₂(Br)(P—CHNP′)][BF4] (4a-d) were synthesized in high yield.Complexes 4a-d were reacted with LiAlH₄, followed by alcohol, togenerate a mixture of iron hydride complexes with proposedmer-Fe(H)(CO)(OR)(P—CH₂NH—P′) structures, where R=Me or tAmyl. A mixtureof monohydride iron pre-catalysts 7a, 7b or (S,S)-8d that had alkylsubstituents on one of the phosphorus atoms of the P—CH₂NH—P′ ligandwere active for ketone hydrogenation and aldehydes under mild basicconditions (T=50° C., p(H₂)=5 atm). Catalytic performance reached TOF upto 2000 h⁻¹, TON up to 2000 and enantioselectivities up to 85% (S).Consequently, it has been shown that a variety of such iron hydridecompounds can be prepared in a few steps from aldehyde and aminebuilding blocks on Fe(II).

Further, trans dihydrido iron complexes 11a and (S,S)-11b wereidentified by experiment, as well as their cis-dihydride isomers 12a and(S,S)-12b. Unlike the trans dihydride FeH₂(CO)(PNP) of Milstein that didnot contain an NH group, complexes 11a and (S,S)-11b had inequivalenthydrides, and reacted with acetophenone at 30° C. Therefore, it has beenpostulated that the NH group may be important in catalytic design forketone hydrogenation. The presence of excess base was also considered tofacilitate catalytic activity, although it was demonstrated thatspecific use of a potassium counter-cation was not critical, aspotassium-2,2,2-cryptand worked as well. Consequently, it was proposedthat base may be needed for formation of an undetected hydridoamidocomplex, 13a, in the presence of otherwise more stable alkoxidecomplexes. These experimental observations, supplemented with DFTcalculations, were used to propose a catalytic cycle for ketonehydrogenation. The hydridoamido complex 13a reacts with dihydrogen toform a reactive trans-dihydride complex, which attacks a ketone in anouter coordination sphere, much like related ruthenium metal-ligandbifunctional systems. A hydride and then a proton are transferredstepwise to produce the product alcohol, and an hydridoamidointermediate 13a or (S,S)-13b. Relative energies of the (S,S)-11bsystem's pro-S and pro-R transition states were calculated using DFT,and were consistent with the observed enantioselectivity favouring the(S) alcohol. Calculations suggested that the energy barriers for hydrideattack on the ketone, and dihydrogen splitting at the amido complex weresimilar, and that either step could be turn-over limiting depending onconditions. In the absence of dihydrogen, the hydridoamido intermediates(13a or (S,S)-13b) decomposed to iron(0) species.

TABLE 1 Hydrogenation of acetophenone catalyzed by achiral complexes4a-b once activated by reaction with LiAlH₄ and then alcohol.^(a)

Pre- Temper- Time cata- ature C/S (min, 99% TOF ^(c) Entry lyst^(a)Base^(b) (° C.) Alcohol^(a) ratio conv) (h⁻¹) 1 4a KOtBu 50 tAmylOH1/500 15 1980 2 4b KOtBu 50 tAmylOH 1/500 15 1980 3 4b KOtBu 25 tAmylOH1/500 30 990 4 4b^(d) KOtBu 50 tAmylOH 1/500 30 990 5 4b KOtBu 50 MeOH1/500 55 550 6 4b KOtBu 50 EtOH 1/500 30 990 7 4b None 50 tAmylOH 1/500No — conver- sion 8 4a KOtBu 50 tAmylOH 1/2000 60 1980 9 4c KOtBu 50tAmylOH 1/500 No — conver- sion ^(a)Pre-catalyst activated in situ: 5 mg(4a, b or c), 4 equiv LiAlH₄ (0.05 mL 1M in THF), followed by 0.5 mLalcohol; 6 mL THF. ^(b)Base (C/B = 1/10) dissolved in 4 mL THF and addedinto reactor preloaded with THF solution with pre-catalyst and substrateto commence catalysis. ^(c) Calculated at 99% conversion. ^(d)NaAlH₄used instead.

TABLE 2 Reactivity of various ketones in the asymmetric hydrogenationreaction using an in situ generated catalyst derived from (S,S)-4d.^(a)

En- Time % e.e.^(b) TOF try Substrate (h) conv.^(b) (%) (h⁻¹) TON  1

 0.5 99 80 (S) 1980 990  2

 1.5 95 80 (S)  630 950  3

 1.3 95 80 (S)  730 950  4

 1 99 76 (S)  990 990  5

 1.5 95 77 (S)  630 950  6

 0.8 96 82 (S) 1200 960  7

 0.6 96 79 (S) 1600 960  8

 0.8 90 83 (S) 1500 900  9

 6 90 60 (S)  150 900 10

12 90 37 (S)  65 800 11

 2 95 n/a  500 950 12

18 92 22 (S)  50 920 13

 0.5 99 80 (S) 1980 990 14

 0.8 99 85 (S) 1230 990 15

 1 97  5 (S)  970 970 16

11 97 30 (S)  90 970 17

 3 91 46 (S)  300 910 18

 4 95 82 (S)  240 950 19

 5 90 85 (S)  220 900 20

 1 20^(c) 74 (S)^(d)  200 200 21

24  0 n/a   0  0 22

 4 95  0  240 950 ^(a)Catalyst prepared in situ: (S,S)-4d (5 mg, 0.007mmol) mixed with 6 equiv LiAlH₄ (0.05 mL of 1M LiAlH₄ in THF) followedby 0.5 mL tAmylOH in 6 mL THF. ^(b)Determined by GC. ^(c)Determined by¹H NMR spectroscopy. ^(d)Determined by HPLC.

TABLE 2a Asymmetric hydrogenation of acetophenone using an in situgenerated chiral catalyst.^(a)

Time e.e.^(b) TOF Entry “Fe” (h) % conv.^(b) (%) (h⁻¹) TON 1 (S,S)-4d0.5 99 80 (S) 1980 990 2 (S)-4e 1 40 63 (S) 400 400 3 (S)-4f 1 99 55 (S)990 990 4 (S)-4g 2 99 13 (S) 495 990 ^(a)Catalyst prepared in situ: “Fe”(0.007 mmol) mixed with 6 equiv LiAlH₄ (0.05 mL of 1M LiAlH₄ in THF)followed by 0.5 mL tAmylOH in 6 mL THF. ^(b)Determined by GC.

TABLE 3 Enantio-determining step (EDS) transition state energiescalculated using different functionals^(a) ΔG_(S) ^(≠) ΔG_(R) ^(≠)ΔG_((R−S)) ^(≠) Functional (kcal/mol) (kcal/mol) (kcal/mol) ee % M11-L19.65 22.14 2.49 97 M06 15.18 17.17 1.99 93 mPW1PW91 25.58 28.09 2.51 97PBE0 22.25 25.41 3.15 99 ωB97X-D 12.01 14.23 2.22 95 ^(a)All of the EDStransition states were calculated using the Berny algorithm methodprovided in Gaussian (opt = (CalcFC, ts)). Initial guess geometries werelocated by optimizing a restricted model where the iron-hydride andhydride-carbonyl carbon distances were fixed; these distances came fromthe corresponding transition state located with a simplified structureor from the EDS TS found using one of the other functionals.

All publications, patents and patent applications mentioned in thisSpecification are indicative of the level of skill of those skilled inthe art to which this invention pertains and are herein incorporated byreference to the same extent as if each individual publication, patent,or patent applications was specifically and individually indicated to beincorporated by reference.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

The invention claimed is:
 1. A complex of formula (I)

wherein: a dashed line indicates that a bond may or may not be present;each R¹ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₁-C₈ alkoxy,C₃-C₁₀ cycloalkyl, aryl, or heteroaryl, each of which may be optionallysubstituted; or the two geminal R¹ substituents combine to form a C₂-C₄linear alkyl diradical or C₃-C₁₀ branched alkyl diradical, each of whichmay be optionally substituted, such that the two R¹ substituents,together with the phosphorus atom to which they are attached, form aring; each R² is independently aryl, heteroaryl, C₁-C₁₀ alkyl, C₂-C₁₀alkenyl, C₁-C₁₀ alkoxy, aryloxy, or C₃-C₁₀ cycloalkyl, each of which maybe optionally substituted; or the two geminal R² groups combine to forma C₂-C₄ linear alkyl diradical or C₃-C₁₀ branched alkyl diradical, eachof which may be optionally substituted, such that the two R²substituents, together with the phosphorus atom to which they areattached, form a ring; R³, R⁴, R⁵, R⁶, R⁸, R⁹, R¹⁰ and R¹¹ are eachindependently H, or C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be optionally substituted; or, R³and R⁴, R⁵ and R⁶, and/or R¹⁰ and R¹¹, together with the carbon atoms towhich they are attached, form a substituted C₅-C₁₀ cycloalkyl ring; R⁷is absent, H, AlH₃, or AlH₅; each L¹ is independently H, BH₄, AlH₄, ahalide, CO, an N-heterocyclic carbene, OR¹², or NCR¹³, wherein R¹² andR¹³ are independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be substituted; or, one of L¹ maybe absent; when R⁷ is AlH₃or AlH₅, at least one of the H may bridge withFe to form a cycle together with the atoms to which they are attached; nis 0, +1, or −1, wherein, when n is +1, the complex further comprises atleast one non-coordinating anion, Y; and, when n is −1, the complexfurther comprises at least one non-coordinating cation, Z; such that thetotal charge of the complex is 0; with the proviso that, when thenitrogen is singly bound to the carbon attached to R⁹, each of R³ to R¹¹are H, one L¹ is CO, and the other two L¹,s are Br, or Br and H, or BH₄and H, then the R¹ and R² substituents cannot all be isopropyl; and withthe proviso that, when the nitrogen is singly bound to the carbonattached to R⁹, each of R³ to R¹¹ are H, R⁷ is absent, one L¹ is absent,one of L¹ is CO, and one of L¹ is H, then the R¹ and R² substituentscannot all be isopropyl.
 2. The complex of claim 1, wherein the complexhas the structure of one of the following formulae: (i) the complex hasthe structure of formula (Ia)

wherein: each R¹ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₁-C₈alkoxy, C₃-C₁₀ cycloalkyl, aryl, or heteroaryl, each of which may beoptionally substituted; or the two geminal R¹ substituents combine toform a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branched alkyl diradical,each of which may be optionally substituted, such that the two R¹substituents, together with the phosphorus atom to which they areattached, form a ring; each R² is independently aryl, heteroaryl, C₁-C₁₀alkyl, C₂-C₁₀ alkenyl, C₁-C₁₀ alkoxy, aryloxy, or C₃-C₁₀ cycloalkyl,each of which may be optionally substituted; or the two geminal R²groups combine to form a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branchedalkyl diradical, each of which may be optionally substituted, such thatthe two R² substituents, together with the phosphorus atom to which theyare attached, form a ring; R³, R⁴, R⁵, R⁶, R⁸, R⁹, R¹⁰ and R¹¹ are eachindependently H, or C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be optionally substituted; or, R³and R⁴, R⁵ and R⁶, and/or R¹⁰ and R¹¹, together with the carbon atoms towhich they are attached, form a substituted C₅- C₁₀ cycloalkyl ring; R⁷is H; R¹² is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl,heteroaryl, or C₃-C₁₀ cycloalkyl, each of which may be substituted; andL¹ is CO, N-heterocyclic carbene, or NCR¹³, wherein R¹³ is independentlyC₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, or C₃-C₁₀ cycloalkyl,each of which may be substituted; (ii) the complex has the structure offormula (lb) or (Ic)

wherein each R¹ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₁-C₈alkoxy, C₃-C₁₀ cycloalkyl, aryl, or heteroaryl, each of which may beoptionally substituted; or the two geminal R¹ substituents combine toform a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branched alkyl diradical,each of which may be optionally substituted, such that the two R¹substituents, together with the phosphorus atom to which they areattached, form a ring; each R² is independently aryl, heteroaryl, C₁-C₁₀alkyl, C₂-C₁₀ alkenyl, C₁-C₁₀ alkoxy, aryloxy, or C₃-C₁₀ cycloalkyl,each of which may be optionally substituted; or the two geminal R²groups combine to form a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branchedalkyl diradical, each of which may be optionally substituted, such thatthe two R² substituents, together with the phosphorus atom to which theyare attached, form a ring; R³, R⁴, R⁵, R⁶, R⁸, R⁹, R¹⁰ and R¹¹ are eachindependently H, or C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be optionally substituted; or, R³and R⁴, R⁵ and R⁶, and/or R¹⁰ R¹¹, together with the carbon atoms towhich they are attached, form a substituted C₅ -C₁₀ cycloalkyl ring; L¹is CO, N-heterocyclic carbene, or NCR¹³, wherein R¹³ is independentlyC₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, or C₃-C₁₀ cycloalkyl,each of which may be substituted; and n is −1, wherein the complexfurther comprises at least one non-coordinating cation Z, and the totalcharge of the complex is0; (iii) the complex has the structure offormula (Id)

wherein: each R¹ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₁-C₈alkoxy, C₃-C₁₀ cycloalkyl, aryl, or heteroaryl, each of which may beoptionally substituted; or the two geminal R¹ substituents combine toform a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branched alkyl diradical,each of which may be optionally substituted, such that the two R¹substituents, together with the phosphorus atom to which they areattached, form a ring; each R² is independently aryl, heteroaryl, C₁-C₁₀alkyl, C₂-C₁₀ alkenyl, C₁-C₁₀ alkoxy, aryloxy, or C₃-C₁₀ cycloalkyl,each of which may be optionally substituted; or the two geminal R²groups combine to form a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branchedalkyl diradical, each of which may be optionally substituted, such thatthe two R² substituents, together with the phosphorus atom to which theyare attached, form a ring; R³, R⁴, R⁵, R⁶, R⁸, R¹⁰ and R¹¹ are eachindependently H, or C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be optionally substituted; or, R³and R⁴, R⁵ and R⁶, and/or R¹⁰ and R¹¹, together with the carbon atoms towhich they are attached, form a substituted C₅- C₁₀ cycloalkyl ring; Xis a halide; L¹ is CO; and n is +1, wherein the complex furthercomprises a non-coordinating anion Y, and the total charge of thecomplex is 0; (iv) the complex has the structure of formula (Ie) or (If)

wherein: each R¹ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₁-C₈alkoxy, C₃-C₁₀ cycloalkyl, aryl, or heteroaryl, each of which may beoptionally substituted; or the two geminal R¹ substituents combine toform a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branched alkyl diradical,each of which may be optionally substituted, such that the two R¹substituents, together with the phosphorus atom to which they areattached, form a ring; each R² is independently aryl, heteroaryl, C₁-C₁₀alkyl, C₂-C₁₀ alkenyl, C₁-C₁₀ alkoxy, aryloxy, or C₃-C₁₀ cycloalkyl,each of which may be optionally substituted; or the two geminal R²groups combine to form a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branchedalkyl diradical, each of which may be optionally substituted, such thatthe two R² substituents, together with the phosphorus atom to which theyare attached, form a ring; R³, R⁴, R⁵, R⁶, R⁸, R¹⁰ and R¹¹ are eachindependently H, or C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be optionally substituted; or, R³and R⁴, R⁵ and R⁶, and/or R¹⁰ and R¹¹, together with the carbon atoms towhich they are attached, form a substituted C₅-C₁₀ cycloalkyl ring; X isa halide; and L¹ is CO, N-heterocyclic carbene, or NCR¹³, wherein R¹³ isindependently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, or C₃-C₁₀cycloalkyl, each of which may be substituted; (v) the complex has thestructure of formula (Ig) or (Ih)

wherein: each R¹ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₁-C₈alkoxy, C₃-C₁₀ cycloalkyl, aryl, or heteroaryl, each of which may beoptionally substituted; or the two geminal R¹ substituents combine toform a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branched alkyl diradical,each of which may be optionally substituted, such that the two R¹substituents, together with the phosphorus atom to which they areattached, form a ring; each R² is independently aryl, heteroaryl, C₁-C₁₀alkyl, C₂-C₁₀ alkenyl, C₁-C₁₀ alkoxy, aryloxy, or C₃-C₁₀ cycloalkyl,each of which may be optionally substituted; or the two geminal R²groups combine to form a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branchedalkyl diradical, each of which may be optionally substituted, such thatthe two R² substituents, together with the phosphorus atom to which theyare attached, form a ring; R³, R⁴, R⁵, R⁶, R⁸, R⁹, R¹⁰ and R¹¹ are eachindependently H, or C₁-C₁₀ alkyl, C₂-C₁₀alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be optionally substituted; or, R³and R⁴, R⁵ and R⁶, and/or R¹⁰ and R¹¹, together with the carbon atoms towhich they are attached, form a substituted C₅-C₁₀ cycloalkyl ring; R⁷is H; and L¹ is CO, N-heterocyclic carbene, or NCR¹³, wherein R¹³ isindependently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, or C₃-C₁₀cycloalkyl, each of which may be substituted; or (vi) the complex hasthe structure of formula (Ii)

wherein: each R¹ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₁-C₈alkoxy, C₃-C₁₀ cycloalkyl, aryl, or heteroaryl, each of which may beoptionally substituted; or the two geminal R¹ substituents combine toform a C₂-C₄linear alkyl diradical or C₃-C₁₀ branched alkyl diradical,each of which may be optionally substituted, such that the two R¹substituents, together with the phosphorus atom to which they areattached, form a ring; each R² is independently aryl, heteroaryl, C₁-C₁₀alkyl, C₂-C₁₀ alkenyl, C₁-C₁₀ alkoxy, aryloxy, or C₃-C₁₀ cycloalkyl,each of which may be optionally substituted; or the two geminal R²groups combine to form a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branchedalkyl diradical, each of which may be optionally substituted, such thatthe two R² substituents, together with the phosphorus atom to which theyare attached, form a ring; R³, R⁴, R⁵, R⁶, R⁸, R⁹, R¹⁰ and R¹¹ are eachindependently H, or C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be optionally substituted; or, R³and R⁴, R⁵ and R⁶, and/or R¹⁰ and R¹¹, together with the carbon atoms towhich they are attached, form a substituted C₅-C₁₀ cycloalkyl ring; R⁷is H or absent; and L¹ is CO, N-heterocyclic carbene, or NCR¹³, whereinR¹³ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be substituted.
 3. The complex ofclaim 1, wherein: (i) each R¹ is independently C₁-C₈ alkyl or C₃-C₈cycloalkyl; or, alternatively C₁-C₄ alkyl or C₃-C₆cycloalkyl; and, eachR² is independently aryl, or heteroaryl; (ii) R³ and R⁶ are eachindependently H, C₁-C₈ alkyl, aryl, or heteroaryl; or, alternatively H,C₁-C₄ alkyl, aryl, or heteroaryl; and, each R⁴, R⁵, R⁸, R¹⁰ and R¹¹ isH; and/or (iii) R¹² is C₁-C₈ alkyl, C₂-C₈ alkenyl, C₃-C₁₀ cycloalkyl,aryl, or heteroaryl; or, alternatively, R¹² is C₁-C₄ alkyl, C₂-C₄alkenyl, C₃-C₆ cycloalkyl, aryl, or heteroaryl.
 4. The complex of claim3, wherein: (i) each R¹ is independently isopropyl or cyclohexyl; and,each R² is phenyl; (ii) R³ and R⁶ are each independently H, methyl,isopropyl, phenyl, or benzyl; and/or (iii) R¹² is methyl, ethyl,t-butyl, or t-amyl.
 5. The complex of claim 1, wherein Z is an alkalimetal cation.
 6. The complex of claim 1, wherein Y is a conjugate baseof a strong acid.
 7. The complex of claim 6, wherein Y is a halide, BF₄⁻, PF₆ ⁻, SbF₆ ⁻, NO₃ ^(−,ClO) ₄ ⁻, CF₃COO⁻, R¹⁴SO₃ ⁻, CF₃SO₃ ⁻, C₆H₅SO₃⁻, p-CH₃C₆H₄SO₃ ⁻, phosphates, TRISPHAT(Δ- or Λ-P(OC₆Cl₄O)₃ ⁻),carboranes, B(R¹⁴)₄ ⁻ or Al(R¹⁴)₄ ⁻, each of which may be substituted,wherein each R¹⁴ is independently an optionally substituted C₁-C₆ alkyl,aryl, heteroaryl, C₆H₃(CF₃)₂ and C₆F₅, halogen, pseudohalogen, C₁-C₈alkoxide, or aryloxide.
 8. The complex of claim 1, wherein carbon C¹, C²or C³, or any combination thereof, is chiral, and the complex isenantiomerically enriched, or a racemate.
 9. The complex of claim 8,wherein the complex has the structure of formula:


10. A process for the preparation of a complex of claim 1, the processcomprising: (i) reacting a phosphine-aldehyde precursor of formula (II)

wherein R¹, R⁸, R¹⁰, and R¹¹ are as defined in claim 1, with aphosphine-amine of formula (III)

wherein R², R³, R⁴, R⁵, and R⁶ are as defined in claim 1, in thepresence of an iron(II) compound, a CO atmosphere, and a strong base, toform a complex of formula (Ie), or to form a mixture of complexes offormula (Ie) and formula (If),

wherein X is a halide, and L¹ is CO; the complex of formula (Ie), or themixture of complexes of formula (Ie) and formula (If), is furtherreacted in the presence of a silver salt, and a CO atmosphere, to form acomplex of formula (1d)

wherein n is +1, which is further reacted in the presence of a reducingagent to form the complex of formula (Ib) and/or (Ic)

wherein R⁹ is H, and n is −1, which are further reacted with an excessof a primary, secondary, or tertiary alcohol, to form the complex offormula (Ia)

wherein R⁷ is H and R¹² is selected from C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl,aryl, heteroaryl, or C₃-C₁₀ cycloalkyl, each of which may besubstituted; and/or (ii) reacting a complex of formula (Ia)

wherein R¹, R², R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹, R₁₀, R¹¹, R¹² and L¹ are asdefined in (i) above, in the presence of a base, and a H₂ atmosphere toform a complex of formula (Ig) and/or a complex of formula (Ih),


11. The process of claim 10, wherein carbon C¹ and/or C² of thephosphine-amine of formula (III) is chiral; and/or carbon C³ of thephosphine-aldehyde precursor of formula (II) is chiral; and, thecompound of which they are a part is enantiomerically enriched, or aracemate.
 12. The process of claim 10, wherein the iron(II) compound isan iron(II) salt; or, an iron(II) complex.
 13. The process of claim 12,wherein: (i) the iron(II) salt is FeBr₂ or FeCl₂; (ii) the iron(II)complex is Fe(CO)₄Br₂; or (iii) the iron(II) complex is Fe(CO)₄Br₂, andFe(CO)₄Br₂ is additionally reacted in the presence of UV radiation toaid in formation of the complex of formula (Ie); or, to form a mixtureof complexes of formula (le) and formula (If).
 14. The process of claim10, wherein: (i) the strong base is KO^(t)Bu; (ii) the silver salt isAgBF₄, and the BF₄ ⁻ anion acts as a non-coordinating counter-ion forthe complex of formula (Id); (iii) the reducing agent is LiAlH₄ orNaAlH₄; and/or (iv) the alcohol is MeOH, EtOH, ^(t)BuOH or ^(t)AmylOH.15. The process of claim 10, wherein carbon C¹, C² or C³, or anycombination thereof, of complexes of formula (Ia), (1g), or (1h) ischiral, and the complex is enantiomerically enriched, or a racemate. 16.The process of claim 10, wherein the base referenced in (ii) is KOtBu,NaOtBu, Ph-CH(OK)CH₃, or NaOMe.
 17. A method for hydrogenation of asubstrate comprising contacting the substrate with a hydrogen source inthe presence of a complex of claim 1, under conditions suitable forhydrogenation.
 18. The method of claim 17, wherein the substrate is aketone, aldehyde, or imine.
 19. The method of claim 17, wherein thehydrogen source is hydrogen gas at a pressure >0 atm and <70 atm. 20.The complex of claim 1, wherein the complex has the structure of one ofthe following formulae: (i) the complex has the structure of formula(Ig) or (Ih)

wherein: each R¹ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₁-C₈alkoxy, C₃-C₁₀ cycloalkyl, aryl, or heteroaryl, each of which may beoptionally substituted; or the two geminal R¹ substituents combine toform a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branched alkyl diradical,each of which may be optionally substituted, such that the two R¹substituents, together with the phosphorus atom to which they areattached, form a ring; each R² is independently aryl, heteroaryl, C₁-C₁₀alkyl, C₂-C₁₀ alkenyl, C₁-C₁₀ alkoxy, aryloxy, or C₃-C₁₀ cycloalkyl,each of which may be optionally substituted; or the two geminal R²groups combine to form a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branchedalkyl diradical, each of which may be optionally substituted, such thatthe two R² substituents, together with the phosphorus atom to which theyare attached, form a ring; R³, R⁴, R⁵, R⁶, R⁸, R⁹, R¹⁰ and R¹¹ are eachindependently H, or C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be optionally substituted; or, R³and R⁴, R⁵ and R⁶, and/or R¹⁰ and R¹¹, together with the carbon atoms towhich they are attached, form a substituted C₅-C₁₀ cycloalkyl ring; R⁷is H; and L¹ is CO, N-heterocyclic carbene, or NCR¹³, wherein R¹³ isindependently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, or C₃-C₁₀cycloalkyl, each of which may be substituted; (ii) the complex has thestructure of formula (Ii)

wherein: each R¹ is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₁-C₈alkoxy, C₃-C₁₀ cycloalkyl, aryl, or heteroaryl, each of which may beoptionally substituted; or the two geminal R¹ substituents combine toform a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branched alkyl diradical,each of which may be optionally substituted, such that the two R¹substituents, together with the phosphorus atom to which they areattached, form a ring; each R² is independently aryl, heteroaryl, C₁-C₁₀alkyl, C₂-C₁₀ alkenyl, C₁-C₁₀ alkoxy, aryloxy, or C₃-C₁₀ cycloalkyl,each of which may be optionally substituted; or the two geminal R²groups combine to form a C₂-C₄ linear alkyl diradical or C₃-C₁₀ branchedalkyl diradical, each of which may be optionally substituted, such thatthe two R² substituents, together with the phosphorus atom to which theyare attached, form a ring; R³, R⁴, R⁵, R⁶, R⁸, R⁹, R¹⁰ and R¹¹ are eachindependently H, or C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be optionally substituted; or, R³and R⁴, R⁵ and R⁶, and/or R¹⁰ and R¹¹, together with the carbon atoms towhich they are attached, form a substituted C₅-C₁₀ cycloalkyl ring; R⁷is absent; and L¹ is CO, N-heterocyclic carbene, or NCR¹³, wherein R¹³is independently C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, aryl, heteroaryl, orC₃-C₁₀ cycloalkyl, each of which may be substituted; or (iii)